Our knowledge of calendars seems to be incomplete though the efforts to design and bring to perfection a sys­tem of calendars took roots in the very ancient times.
Perhaps as early as pre-historic days, even before man knew reading or writing, he might have divined some system of scratching on bone plaques or etching on rocks to keep count of days divided into periods matching the lunar phases. The earliest organ­ized system of calendars was the one created by reformation of the Roman calendar by Julius Caesar, in 46 BC. This lasted for about 1600 years, (the year as we know now). This was sub­sequently reformed again, in 1582 AD, by Pope Gregory XIII, to give the world the Gregorian calendar, which is now accepted by almost all na­tions, to connote the dates uni­formly. However there are over 40 different systems of calendars preva­lent all over the world

In addition to above there has been the concept of the 'week', though not associated with any known astro­nomical phenomena; this has been one of the oldest surviving human institutions with a history of continu­ous observation over 3000 years. The number of days in a week varied from society to society - 5, 7, 8, 9, 10 days. The seven-day week was ob­served by the Babylonians, Jews and some nations in West Africa and has come to stay till date. These were named after the then known seven celestial objects - the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn - all visible to the human eye. This could perhaps be due to the in­fluence of people with an astrological bent from Babylon or the Roman world or the Orientals from Asia to whom the number seven was a kind of mystical, if not lucky, number. The number seven is seen to be asso­ciated with several well known cliches - seven ages of man, seven wonders of the world, seven continents of the world, Snow white and the seven dwarfs, seven stars of the constella­tion commonly known as the Great Bear (Sapta Rishis), Sapta Swaras, Sapta Thandava (seven dances of Shiva), Sapta Loka (seven worlds), Sapta saagara (seven seas).
Recently chrono-biologists have dis­covered that the seven-day cycle, like the day-night sleep cycle may also have biological precedents. They say that certain biorhythms of the body, like variations in heart­beat, blood pressure and response to infection, work on seven-day cycles. A practical geometrical theory as well - if you wrap a rubber band around 7 soda cans (or any other con­venient cylindrical objects), you get a perfect hexagon with the 7th can in the middle. It is the only stable con­figuration of wrapping more than 3 cylindrical objects. 4, 5, and 6 objects will slip from one configuration to another. Ancients wrapping tent poles, small logs for firewood or other circular objects might have come upon this number and attach a mystical significance to it.
Changes in the appearances of  the moon also provided another possible reason for the seven-day week. Originating with ancient interpretations of lunar time, divisions of seven-days separate the four basic phases of the Moon, viz New Moon, First Quarter, Full Moon, Fourth Quarter. Starting with a dark New Moon, the Moon gradually comes into view on following nights. In about seven-days, the first half of the Moon is visible. The Moon waxes until Full Moon at the end of two weeks. Lunar light reverses progression in the third week, waning to half ­visibility again. A fourth week completes the month, and visibility again ­diminishes toward a New Moon. Completion of four lunar phases comprises the month. The true lunar month is measured precisely to be 29.53 days. By actual observations ancient calendar makers recorded approximations. The substitute lunar month of 29.5 days was determined upon sighting the new-moon crescent. There is no record of the 7-day week cycle ever having been broken. Calendar changes and reform have never interrupted the 7-day cycles. It is very likely that the week cycles have run uninterrupted at least since the  days of Moses (1400 BC.) or possibly even longer.
Astrology was perhaps also responsible for another curious thing in our weekly calendar - the order of the days. We take the present order Sunday, Monday, Tuesday, etc. for granted. In fact it does not corre­spond to our understanding of the solar system. \

According to distances of the seven heavenly objects from the Earth, in descending order, they are - Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. The reason for the discrepancy between this order and the days of week fol­lowed by us seems to come from an invention from Mesopotamia - the division of the day into 24 equal units of time. Ancient Mesopotamian as­trologers attache a planet-god to preside over each hour of the day, arranged according to their correct cosmological order.

For example, Saturn controlled the first hour of Saturn's day (Saturday), followed by Jupiter in its second hour, then by Mars, the Sun, Venus, Mercury and the Moon. In the eighth hour the cycle again started with Saturn until the twenty fourth hour of the day by Mars. The next hour of the cycle, (i.e.) the first hour of the next day, belonged to the Sun, and, there­fore, the day after Sat­urday was called Sunday.

Our ancestors, perhaps as early as some 10,000 generations back, needed some form of a timetable of events to find the local food or to migrate themselves to avoid the cold winter; and, for this purpose watched the Sun, the Moon and the stars for clues. They considered the repetition of the seasons, the regular flooding of the rivers, the fall of yellowed leaves and the blossoming of fresh buds were due to the unseen powers of gods; and, they thought perhaps the Sun, the Moon and the planets were associated with gods. This led on to giving birth to cultures develop­ing alliance between religion and as­trology and in time giving rise to as­tronomy as we know today.

Calendar systems
There are over 40 systems of calen­dars in vogue today around the world, observed by various groups of people belonging to various religions and regions of nations in order to observe their fasts and feasts of their religious order. Some of these are: Alexandrian, Aztec, Babylonian, Ba­ha’i, Celtic, Chinese, Egyptian, Ethio­pian, Fasli, French, Gregorian, Ice­landic, Indian, Islamic, Japanese, Jewish, Julian, Macedonian, Mayan, Neolithic, Ottoman (financial), Per­sian, Roman, Saka and Soviet. Calendars can be classified as: arith­metic, astronomical, calculated, em­pirical, lunar, lunisolar and regular.

Roman calender
The Romans borrowed parts of their earliest known calendar from the Greeks. The early Roman calendar originated as a local calendar in the city of Rome, suppos­edly drawn up by Romulus some seven or eight centuries before the Christian Era. The year began in March and consisted of 10 months, six of 30 days and four of 31 days, making a total of 304 days: it ended in December, to be followed by what seems to have been an uncounted winter gap. Later around 700 B.C. two extra months, January and Feb­ruary, were added to fill the gap and to have increased the total number of days by 50, making 354. To obtain sufficient days for his new months, he is then said to have deducted one day from the 30-day months, thus having 56 days to divide between January and February. But since the Romans had, or had developed, a superstitious dread of even numbers, January was given an extra day; Feb­ruary was still left with an even num­ber of days, but as that month was given over to the infernal gods, this was considered appropriate. The sys­tem allowed the year of 12 months to have 355 days, an uneven number.

Gregorian calendar
The Julian calendar had several defects; the average number of days in its year was 365.25 whereas the true tropical year was about 365.24219, thus causing vernal equinox falling earlier and earlier and the date of Easter falling later and later. So when Gregory XIII succeeded as Pope he undertook to reform the calendar. In the year 1582 AD he de­creed that the day after October 4, 1582 would be October 15, 1582, by dropping out 10 days from the face of the calendar. He also decreed that leap years are omitted in years divisible by 100 but not divisible by 400. By this rule, the year 1900 was not a leap year (1900 is divisible by 100 and not divisible by 400), but the year 2000 will be a leap year (2000 is divisible by 400). The total number of days in 400 years is therefore given by 400*365 + 100­3=146,097. This also gives an exact num­ber of 146097/7=20,871 weeks per 400­year cycle. The Gregorian calendar was constructed to give a close approximation to the tropical year which is the actual length of time it takes for the Earth to complete one orbit around the Sun. Some of the catholic coun­tries accepted this change. Various Catholic German countries (Germany was not yet unified), Belgium, the Netherlands, and Switzerland followed suit within a year or two, and Hungary followed in 1587. How­ever, the rest of Europe did not follow suit for more than a century.
The Protestant German countries adopted the Gregorian reform in 1700. By this time, the calendar trailed the seasons by 11 days. England (and the American colonies) finally followed suit in 1752, and Wednes­day, September 2, 1752 was immediately followed by Thursday, September 14, 1752. This traumatic change resulted in widespread riots and the populace demand­ing "Give us the eleven days back'" English Calendar then was:

Sweden followed England's lead in 1753. Russia, however, did not follow suit until 1918, when January 31,1918 was immedi­ately followed by February 14th. [In fact, however, the (then) USSR is not on the Gregorian calendar, but on a more accurate one of their own devising. The (then) USSR calendar is designed to more closely approximate the true length of the tropical year, thus has one additional rule for when a year is a leap year. It will remain in synchronization with the Gregorian calen­dar for thousands more years, by which time one or both will have probably fallen into disuse.

Islamic calendar
The Islamic calendar has its starting point at the date of the flight of Mohammed from Mecca to Medina, known as the Hegira. The most widely accepted date for this event in the Gregor­ian calendar is sunset at July 16, 622 AD. This dating system is used in the Muslim world (except Turkey, which uses the Gregorian calendar) and based on a year of 12 months, each month beginning approxi­mately at the time of the New Moon. (The Iranian calendar, however, is based on a solar year.)
The Islamic calendar is tied to the lunar phase cycle, with each month alternatively having either 29 or 30 days, except for the 12th, the length of which is varied in a 30­year cycle intended to keep the calendar in step with the true phases of the Moon. The calendar therefore drifts by 365.24-354 = 11.24 days relative to the sun each Islamic year, and would require 365.24/11.24 Islamic years to get back in synchroniza­tion. As a result, the calendar uses an 11­year leap year cycle. In 11 years of this cycle the 12th month has 30 days and in the other 19 years it has 29. Thus the year has either 354 or 355 days. No months are ever added, so that the named months do not remain in the same seasons but retrogress through the entire solar, or seasonal, Year (of about 365.25 days) every 32.5 solar years.

Indian calendar
As a result of a calendar reform in 1957 A.D., the National Calendar of India is a formalized lunisolar calendar in which leap years coincide with those of the Gregorian calendar. However, the initial epoch is the Saka Era, a traditional epoch of Indian chronology. Months are named after the traditional Indian months and are offset from the beginning of Gregorian months (see table 2 below).
In addition to establishing a civil calendar, the Calendar Reform Committee set guide lines for religious calendars, which require calculations of the motions of the Sun and Moon. Tabulations of the religious holidays are prepared by the India Meteorological Department and published annually in The Indian Astronomical Ephemeris.

The sequence of months and their correla­tion with the months of the Gregorian calendar are as given in the table below. The history of calendars in India is a remarkably complex subject owing to the continuity of Indian civilization and to the diversity of cultural influences. In the mid­1950s, when the Calendar Reform Commit­tee made its survey, there were about 30 calendars in use for setting religious festi­vals for Hindus, Buddhists, and Jains. In addition, Muslims in India used the Islamic calendar, and the Indian government used the Gregorian calendar for administrative purposes.
Early allusions to a lunisolar calendar with intercalated months are found in the hymns from the Rig Veda, dating from the second millennium B.C. Literature from 1300 B.C. to A.D. 300, provides information of a more specific nature. Indian astronomy underwent a general reform in the first few centuries of C.E., as advances in Babylo­nian and Greek astronomy became known. "Jew astronomical constants and models for the motion of the Moon and Sun were adapted to traditional calendric practices. This was conveyed in astronomical trea­tises of this period known as Siddhantas, many of which have not survived. The Slirya Siddhanta, which originated in the fourth century but was updated over the following centuries, influenced Indian calendrics up to and even after the calendar reform of A.D. 1957.

Religious holidays are determined by a lunisolar calendar that is based on calculations of the actual positions of the Sun and Moon. Most holidays occur on specified lunar dates (tithis), as is explained later; a few occur on specified solar dates. The calendrical methods presented here are those recommended by the Calendar Reform Committee (1957). They serve as the basis for the calendar published in The indian Astronomical Ephemeris. However, many local calendar makers continue to use traditional astronomical concepts and for­mulas, some of which date back 1500 years.
The Calendar Reform Committee attempted to reconcile traditional calendri­cal practices with modern astronomical concepts. According to their proposals, precession is accounted for and calcula­tions of solar and lunar position are based on accurate modern methods. All astro­nomical calculations are performed with respect to a Central Station at longitude 82° 30' east, latitude 23011' north. For religious purposes solar days are reckoned from sun­rise to sunrise.
A solar month is defined as the interval required for the Sun's apparent longitude to increase by 30°, corresponding to the passage of the Sun through a zodiacal sign (rasi). The initial month of the year, Vaisakha, begins when the true longitude of the Sun is 230 IS' (see Table below). Because the Earth's orbit is elliptical, the lengths of the months vary from 29.2 to 31.2 days. The short months all occur in the second half of the year around the time of the Earth's perihelion passage.
Apart from the Indian National calendar there are still a number of calendars in several linguistic regions (states) of the country. Solar calendars are in vogue in Assam, Haryana, Kerala, Orissa, Punjab, Tamil Nadu and West Bengal. LuniSolar calendars are in vogue in Andhra Pradesh, Bihar, Gujarat, Jammu & Kashmir, Karna­taka, Madhya Pradesh, Maharashtra, Rajasthan and Uttar Pradesh. The feasts and fasts of the Hindus are determined by these calendars, also known as almanac or panchang.

Chinese calendar
Although the People's Republic of China uses the Gregorian calendar for civil purposes, a special luni­solar calendar is used for determining festi­vals. Various Chinese communities around the world also use this calendar. The beginnings of the Chinese calendar can be traced back to the 14th century B.C. The Chinese calendar is based on exact astronomical observations of the longitude ofthe sun and the phases of the moon. This means that principles of modern science have had an impact on the Chinese calendar.
The Chinese calendar is a combined solari lunar calendar in that it strives to have its years coincide with the tropical year and its months coincide with the synodic months. It is not surprising that a few similarities exist between the Chinese and the Hebrew calendar: An ordinary year has 12 months; a leap year has 13 months. An ordinary year has 353, 354, or 355 days, a leap year has 383, 384, or 385 days.
Unlike most other calendars, the Chinese calendar does not count years in an infinite sequence. Instead years have names that are repeated every 60 years.

Possible future development

The dominant conceptual scheme for civil time-keeping at present is the Gregorian calendar: a 400-year-old modification of a 2000-year-old scheme known as the Julian calendar. Time-keeping and scheduling in our present, post-industrial, information­age society thus rely on an anachronistic scheme serving the interests of men in a pre-scientific, theocratic society, with a feudal economy. The invention of mechanical clocks made it possible to divide every day into twenty ­four equal time-segments. The day is there­fore easily divisible into halves, thirds and quarters, as is each of its twenty-four hours. The practical advantage of these regular divisions over the variable divisions of daylight, from dawn to noon to sunset, is obvious.
In contrast, the Gregorian calendar's strict adherence to the solar cycle produces an expiring calendar every year. This requires continual schedule-revisions for many important activities, such as education. It also precludes regular divisions within the year necessary for accurate statistical comparisons. Half-years have an equal number of days only in leap-years; the year never divides evenly into quarters; the months are irregular; and neither the year nor the months can be divided regularly into weeks.
The World Calendar is a 12-month, peren­nial calendar with equal quarters. It is perennial because it remains the same every year. In the World Calendar every year is the same. The quarters are equal: each has exactly 91 days, 13 weeks or 3 months; the quarters are identical in form with an ordered variation within the three months. The three months have 31, 30, 30 days respectively. Each month has 26 week­days, plus Sundays. Each year begins on Sunday 1 January; each working year begins on Monday 2 January. Each quarter begins on Sunday, ends on Saturday. The calendar is stabilized and made perpet­ual by ending the year with a 365th day following 30 December each year. This additional day is dated 'W,' which equals 31 December, and ca11ed Worlds day, a year-end world holiday. Leap-year Day is similarly added at the end of the second quarter. It is likewise dated 'W,' or 31 June, and called Leap year Day, another world holiday in leap years.
Our present calendar is not perennial, but annual. It changes every year. It does so because its typical 365-day cycle is not evenly divisible by the number of days in the week: 365 ~ 7 = 52, remainder 1. The unfortunate consequence of that one-day remainder is that the year typically begins and ends on the same weekday. So the next year must begin on the fo11owing weekday. This requires a new calendar every year.

Advantages of the World Calendar ­
Numbered days of the month always fall on the same weekdays: so the birthday of Tuesday's Child is always on a Tuesday. No need to schedule events by cumber­some weekday-and-month designations, like US Election Day, "First Tuesday after the First Monday in November": Election Day wi11 always be Nov. 7; Thanksgiving (US) will always be on November 23rd.
The year divides regularly into quarters of equal size (91 days or 13 weeks or 3 months), the same number of workdays (65) and weekend-days (26) in each quarter - A great improvement over the Gregorian calendar for statistical comparisons be­tween quarters. The variations in month ­length are more regular than the Gregorian calendar: most months have 30 days; the first months of the quarters (Jan, April, July, Oct) have 31. Excluding Sundays, all months have the same number of days: 26.
Transition from the Gregorian calendar would be extremely simple: reform couk be instituted in 2006 (a Gregorian year beginning on Sunday) and only a few dates in February, March, April, May, August and December would be affected.
Some Disadvantages of the World Calendar - Religious groups obliged to worship every seven days will have a problem with off-calendar days: sometimes there will be seven days between two occurrences of the weekday they choose to worship.
There are FOUR Fridays, the 13th EVERY YEAR
There are several other such proposals from various calendar associations around the world and there are, as usual, objections galore for each one of them. It is, however unlikely that the present system of interna­tional calendar may not see a change in the future. Having said so much there is still so much more to say about the mysteries of one of the greatest of man's inventions, the calendar system.