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Living: Calendar

All three religions work to the same seven-day week. However, their systems of years and dates differ.


Gregorian ('Christian') Calendar

The Western Christian world uses the Gregorian calendar, developed in the 16th century, for expressing dates and setting festivals. Its adoption by many non-Christian countries over the next 300 years has made it the most widespread system. (Orthodox Christianity did not adopt the Gregorian calendar and has kept to the older Julian calendar, currently 13 days behind.)

It is based on movements of the earth around the sun. This means sun-related events such as the shortest day or equinoxes fall around the same date every year. Each year has 12 months totalling either 365, or (once every four years) 366 days.

Christmas is celebrated on 25 December in Western Christianity, and 7 January in Orthodox. Easter is a 'movable feast', varying with the moon's phases. In Western Christianity, it is the first Sunday after the notional full moon that follows 14 days after the first new moon after 21 March. Easter 2007 was 8 April in both Western and Eastern churches; in 2008 it will be 23 March in Western, and 27 April in Eastern.

The current year of 2007 is nominally based on Jesus being born in the year 1. Years are designated with the letters AD (anno Domini, 'year of the Lord' in Latin); or, for years before that, BC (before Christ). In multi-faith or non-religious contexts, CE (common era) and BCE (before common era) often replace AD and BC respectively.


Jewish calendar

In Judaism, the Jewish calendar is used to determine the dates of holidays and festivals, or appropriate portions of religious text for reading. It is based on the relative positions of the earth, moon and sun. Each year has 12 regular months, with seven extra months inserted at various times during a 19-year cycle. This makes it slower than the Gregorian system by one day every 231 years.

The Jewish calendar is complex. Years come in 14 different month-day layouts, characterised by the date on which the festival of Rosh Hashanah (New Year) falls. The number of the Jewish year is based on 1 as the year of creation. For example, Monday 3 September 2007 in the Gregorian calendar was 20 Elul 5767 in the Jewish calendar. The year number may be clarified by the letters AM (anno mundi, 'year of the world' in Latin), for example 5767AM.


Islamic calendar

The Islamic calendar is used to determine the dates of Muslim holy days and festivals. It is based on movements of the moon round the earth. Each year of 12 lunar months has about 354 days (the Qur'an forbids the insertion of extra months) and thus moves out of step with the Gregorian calendar by 11 days or so annually.

Sun-related events, such as the shortest day or the equinoxes, therefore occur later in the Muslim calendar every year by 11 days. The holy month of Ramadan occupies a different part of the Western calendar every year. Thus a Muslim living in a northern country might have only short winter daylight hours to fast one year; however, they would have long summer daylight hours to contend with a decade and a half later.

The number of the Islamic year is based on the hijra, Muhammad's flight from Mecca to Medina, which was in year 1. For example, Monday 3 September 2007 in the Gregorian calendar was 20 Shaban 1428 in the Islamic calendar. The year number may be clarified by the letters H or AH (anno Hegirae, 'year of hijra' in Latin), for example 1428AH.

Western-style calendar

Jewish calendar

Islamic calendar