This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The "Christian calendar" is the term traditionally used to designate
the calendar commonly in use, although its connection with
Christianity is highly debatable.
The Christian calendar has years of 365 or 366 days. It is divided into
12 months that have no relationship to the motion of the moon. In
parallel with this system, the concept of "weeks" groups the days in
sets of 7.
Two main versions of the Christian calendar have existed in recent
times: 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.