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cal·en·dar (kălən-dər)
1. Any of various systems of reckoning time in which the beginning, length, and divisions of a year are defined, sometimes along with multiyear cycles.
2. A table showing the months, weeks, and days in at least one specific year.
3. A schedule of events.
4. An ordered list of matters to be considered:the bills on a legislative calendar.
5. Chiefly British A catalog of a university.
tr.v. cal·en·dared, cal·en·dar·ing, cal·en·dars
To enter in a calendar; schedule.

[Middle English calender, from Old French calendier, from Late Latin kalendārium, from Latin, account book, from kalendae, calends (from the fact that monthly interest was due on the calends); see kelə-2 in the Appendix of Indo-European roots.]


The Gregorian calendar is now in use as the civil calendar throughout most of the world. The Jewish calendar is the official calendar of the Jewish religious community. The Islamic calendar is the official calendar in many Muslim countries. Each calendar listed below begins with the first month of the year and includes the number of days each month contains. Many months have a variable number of days, as described below.

The Gregorian calendar, introduced in 1582 by Pope Gregory XIII, is a corrected form of the Julian calendar. It is based on a solar year of 365 days. Every fourth year is a leap year of 366 days except for centenary years not evenly divisible by 400.The Jewish year consists of twelve months defined by lunar cycles, with some years having a thirteenth month so that seasonal festivals stay aligned with the solar year. For religious purposes Nisan is the first month, but the New Year is celebrated in Tishri. In leap years Adar is known as Adar Sheni.The Islamic calendar is based on the lunar year and contains 354 or 355 days. The number of days in each month varies with the lunar cycle. The beginning of the year retrogresses through the solar year, completing a full cycle every 32.5 years.
MonthsNumber of DaysMonthsNumber of DaysMonthsNumber of Days
January 31Nisan (Mar-Apr)30Muharram 29 or 30
February 28 or 29Iyar (Apr-May)29Safar 29 or 30
March 31Sivan (May-Jun)30Rabi I 29 or 30
April 30Tammuz (Jun-Jul)29Rabi II 29 or 30
May 31Av (Jul-Aug)30Jumada I 29 or 30
June 30Elul (Aug-Sep)29Jumada II 29 or 30
July 31Tishri (Sep-Oct)30Rajab 29 or 30
August 31Heshvan (Oct-Nov)29 or 30Shaʾban 29 or 30
September 30Kislev (Nov-Dec)29 or 30Ramadan 29 or 30
October 31Tevet (Dec-Jan)29Shawwal 29 or 30
November 30Shevat (Jan-Feb)30Dhu'l-Qa'dah 29 or 30
December31Adar Rishon(leap year only)30Dhu'l-Hijjah 29 or 30
Adar (Feb-Mar)29

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.