GENESIS 1:14 And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:

The variations among the many calendars in use from ancient to modern times have been caused by the inaccuracy of the earliest determinations of the duration of the year, together with the fact that a year cannot be divided evenly by any of the other time units: days, weeks, or months. The earliest calendars based on lunar months eventually failed to agree with the seasons. A month occasionally had to be intercalated, or added, to reconcile lunar months with the solar year. A calendar that makes periodic adjustments of this kind is a lunisolar calendar.

The original Roman calendar, introduced about the 7th century BC , had 10 months with 304 days in a year that began with March. Two more months, January and February, were added later in the 7th century BC , but because the months were only 29 or 30 days long, an extra month had to be intercalated approximately every second year. The days of the month were designated by the awkward method of counting backward from three dates: the calends, or first of the month; the ides, or middle of the month, falling on the 13th of some months and the 15th of others; and the nones, or 9th day before the ides. The Roman calendar became hopelessly confused when officials to whom the addition of days and months was entrusted abused their authority to prolong their terms of office or to hasten or delay elections. There were calls for a special prosecutor:)

In 45 BC Julius Caesar, upon the advice of the Greek astronomer Sosigenes, decided to use a purely solar calendar. This calendar, known as the Julian calendar, fixed the normal year at 365 days, and the leap year, every fourth year, at 366 days. Leap year is so named because the extra day causes any date after February in a leap year to "leap" over one day in the week and to occur two days later in the week than it did in the previous year, rather than just one day later as in a normal year. The Julian calendar also established the order of the months and the days of the week as they exist in present-day calendars. In 44 BC Julius Caesar changed the name of the month Quintilis to Julius (July), after himself. The month Sextilis was renamed Augustus (August) in honor of the Roman emperor Caesar Augustus, who succeeded Julius Caesar. Some authorities maintain that Augustus established the length of the months we use today.

The Julian year was 11 min and 14 sec longer than the solar year. This discrepancy accumulated until by 1582 the vernal equinox occurred 10 days early and church holidays did not occur in the appropriate seasons. To make the vernal equinox occur on March 21, as it had in AD 325, the year of the First Council of Nicaea, Pope Gregory XIII issued a decree dropping 10 days from the calendar. To prevent further displacement he instituted a calendar, known as the Gregorian calendar, that provided that century years divisible evenly by 400 should be leap years and that all other century years should be common years. Thus, 1600 was a leap year, but 1700 and 1800 were common years.

The Gregorian calendar, or New Style calendar, was slowly adopted throughout Europe. It is used today throughout most of the Western world and in parts of Asia. When the Gregorian calendar was adopted in Great Britain in 1752, another correction of an 11-day discrepancy was made; the day after September 2, 1752, became September 14. The British also adopted January 1 as the day when a new year begins. The Soviet Union adopted the Gregorian calendar in 1918, and Greece adopted it in 1923 for civil purposes, but many countries affiliated with the Greek church retain the Julian, or Old Style, calendar for the celebration of church feasts.

The Gregorian calendar is also called the Christian calendar because it uses the birth of Jesus Christ thought in 1 BC, as a starting date. Dates of the Christian era (see Chronology) are often designated AD (Latin: anno domini, "in the year of our Lord") and BC (before Christ).

As indicated, the Gregorian calendar is basically a Christian calendar. The official Christian church calendar is a table containing the holy days, saints' days, and festivals of the church, with the dates of the civil calendar on which they occur. These include the fixed feasts, such as Christmas, and the movable feasts, which depend on the date of Easter. The most important early church calendar was compiled by Furius Dionisius Philocalus about 354. After the Reformation, the German Lutheran church retained the Roman calendar, as did the Church of England and some other Anglican churches. The calendar of the Protestant Episcopal church retains only those festivals that have a scriptural origin. The principal seasons of the church calendar observed by most Christians are, in order, Advent, Christmas, Epiphany, Lent, Easter, Ascension, Pentecost, and Trinity.

The Jewish calendar is also based on religious doctrine. Only the start of a new year on the first and second day of the month Tischri (September/October) was not set fixed until 70AD. In EXODUS 12, 2 God says to Moses and Aaron: "This month shall be unto you the beginning of months: it shall be the first month of the year to you", and he meant the month Abib (March/April) which was later called Nisan. But EXODUS 34, 22 says: "And thou shalt observe the feasts of weeks, of the firstfruits of wheat harvest, and the feast of ingathering at the year's end." Obviously two different "new-year" events, one in Spring, one in Autumn.

The Jewish calendar, derived from the ancient Hebrew calendar, has remained unchanged since about AD 900. It is the official calendar of the modern state of Israel and is used by Jewish people throughout the world as a religious calendar. Computer translated dates for the year 1752 C.E. and before may not be accurate. They take not into account a correction of twelve days which was introduced by Pope Gregory XIII known as the Gregorian Reformation. The starting point of Hebrew chronology is the year 3761 BC, the date for the creation of the world as described in the Old Testament. The Jewish calendar is lunisolar, based on lunar months of 29 days alternating with 30 days. An extra month is intercalated every 3 years, based on a cycle of 19 years. Dates of the Jewish calendar are designated AM (Latin: anno mundi, "the year of the world") and BCE (before the common era).

Source ©Microsoft Encarta, with additions by Friedhelm Dohmann.

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