ETYM Old Eng. kalender, calender, from Latin kalendarium an interest or account book (cf. French calendrier, Old Fren. calendier) from Latin calendue, kalendae, calends. Related to Calends.
(Homonym: calender).
1. A list or register of events (appointments or social events or court cases etc).
2. A system of timekeeping that defines the beginning and length and divisions of the year.
3. A tabular array of the days (usually for one year).
System determining the length, starting point and subdivisions of the year; table, chart, etc. showing days, weeks and months of a particular year; schedule of events, engagements, etc.
Division of the year into months, weeks, and days and the method of ordering the years. From year one, an assumed date of the birth of Jesus, dates are calculated backward (BC “before Christ” or BCE “before common era”) and forward (AD, Latin anno Domini “in the year of the Lord”, or CE “common era”). The lunar month (period between one new moon and the next) naturally averages 29.5 days, but the Western calendar uses for convenience a calendar month with a complete number of days, 30 or 31 (Feb has 28). For adjustments, since there are slightly fewer than six extra hours a year left over, they are added to Feb as a 29th day every fourth year (leap year), century years being excepted unless they are divisible by 400. For example, 1896 was a leap year; 1900 was not. 1996 is the next leap year.
The month names in most European languages were probably derived as follows: January from Janus, Roman god; February from Februar, Roman festival of purification; March from Mars, Roman god; April from Latin aperire, “to open”; May from Maia, Roman goddess; June from Juno, Roman goddess; July from Julius Caesar, Roman general; August from Augustus, Roman emperor; September, October, November, December (originally the seventh–tenth months) from the Latin words meaning seventh, eighth, ninth, and tenth, respectively.
The days of the week are Monday named for the Moon; Tuesday from Tiu or Tyr, Anglo-Saxon and Norse god; Wednesday from Woden or Odin, Norse god; Thursday from Thor, Norse god; Friday from Freya, Norse goddess; Saturday from Saturn, Roman god; and Sunday named for the Sun.
All early calendars except the ancient Egyptian were lunar. The word calendar comes from the Latin Kalendae or calendae, the first day of each month on which, in ancient Rome, solemn proclamation was made of the appearance of the new moon.
The Western or Gregorian calendar derives from the Julian calendar instituted by Julius Caesar 46 BC. It was adjusted by Pope Gregory XIII 1582, who eliminated the accumulated error caused by a faulty calculation of the length of a year and avoided its recurrence by restricting century leap years to those divisible by 400. Other states only gradually changed from Old Style to New Style; Britain and its colonies adopted the Gregorian calendar 1752, when the error amounted to 11 days, and 3 Sept 1752 became 14 Sept (at the same time the beginning of the year was put back from 25 March to 1 Jan). Russia did not adopt it until the October Revolution of 1917, so that the event (then 25 Oct) is currently celebrated 7 Nov.
The Jewish calendar is a complex combination of lunar and solar cycles, varied by considerations of religious observance. A year may have 12 or 13 months, each of which normally alternates between 29 and 30 days; the New Year (Rosh Hashanah) falls between 5 Sept and 5 Oct. The calendar dates from the hypothetical creation of the world (taken as 7 Oct 3761 BC).
The Chinese calendar is lunar, with a cycle of 60 years. Both the traditional and, from 1911, the Western calendar are in use in China.
The Muslim calendar, also lunar, has 12 months of alternately 30 and 29 days, and a year of 354 days. This results in the calendar rotating around the seasons in a 30-year cycle. The era is counted as beginning on the day Mohammed fled from Mecca AD 622.

To enter into a calendar.