The "Christian calendar" is the term traditionally used to designate the calendar commonly in use today, which starts counting from the approxiate year of Christ's birth. Most nations use this calendar for civil purposes. Christian-oriented nations use the Gregorian and/or Greek Orthodox (Paleoimerologites) calendars for religious purposes.

The Christian calendar has years of 365 or 366 days. Years are divided into 12 months that have no relationship to the cycles of the moon. The year is also divided into approximately 52 weeks of 7 days each.

Two main calendars have been used by Europeans in the past two millennia: the Julian calendar and the Gregorian calendar. The difference between them lies in the way they approximate the length of the tropical year and their rules for calculating Easter.

The Julian calendar was introduced by Julius Caesar in 46 BC. The Julian calendar was naturally adopted by the Christian successors of the Roman Empire. By about 700 CE it had become customary to count years from the starting point of the birth of Christ (later corrected by Johannes Kepler to 4 BCE). But the equinox kept slipping backwards on the calendar one full day every 130 years. By 1500 the vernal equinox fell on the 10th or 11th of March and the autumnal equinox on the 13th or 14th of September, and the situation was increasingly causing unrest among the population.

The most important feast day on the Christian calendar is Easter, when the suffering, death, and resurrection of Christ are celebrated. The New Testament states that Christ's crucifixion occurred in the week of Passover. On the Jewish calendar, Passover was celebrated at the full moon of the first month (Nissan) of spring. In developing their own calendar (4th century CE), Christians put Easter on the first Sunday after the first full moon after the spring equinox. If the equinox fell on the wrong date, then Easter might also be celebrated on the wrong day. Most other Christian observances (such as the beginning of Lent and Pentecost) are reckoned backward or forward from the date of Easter. An error in the equinox thus introduced numerous errors in the entire religious calendar. Something had to be done.

The Gregorian calendar, which is still in use today, was proposed by Rome to correct the errors in the Julian calendar, and was initially adopted in Catholic countries. It 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. The Gregorian calendar currently in worldwide use for secular purposes is based on a cycle of 400 years comprising 146,097 days, giving a year an average length of 365.2425 days. The Gregorian calendar is a modification of the Julian calendar in which leap years are omitted in years divisible by 100 but not divisible by 400. For example, the year 1900 was not a leap year (1900 is divisible by 100 but not divisible by 400), but the year 2000 was a leap year (2000 is divisible by 400).

The Julian calendar was changed to the Gregorian starting in 1582, at which point the 10-day difference between the actual time of year and traditional time of year was deleted. The switchover was bitterly opposed by much of the population, who feared it was an attempt by landlords to cheat them out of a week-and-a-half's rent. However, when Pope Gregory XIII decreed that the day after October 4, 1582 would be October 15, 1582, the Catholic countries of France, Spain, Portugal, and Italy complied. 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.

Because of the Pope's decree, the reformed Julian calendar came to be known as the Gregorian calendar. However, the rest of Europe did not adopt the new calendar 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 switched to the new calendar in 1752, and Wednesday, September 2, 1752 was immediately followed by Thursday, September 14, 1752. This traumatic change resulted in widespread riots and people demanding "Give us the eleven days back!"

English calendar:

Sweden followed England's lead in 1753. Russia, however, did not follow suit until 1918, when January 31, 1918 was immediately followed by February 14. In fact, however, Russia is not on the Gregorian calendar, but on a more accurate one of their own devising. The Russian Orthodox 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 of years, by which time one or both will have probably fallen into disuse. Similarly, the Iranian calendar is also a more accurate version of the Gregorian calendar.

For your information, the Gregorian calendar is useless for astronomy because of its 10 missing days. For the purpose of calculating positions backward in time, astronomers use the Julian dates.

Please NOTE that the above information is just a brief explanation of the Gregorian calendar. More information can be found at libraries.

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