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 Islamic calendar Months of Islamic calendar Islamic calendar, also called Hijrī calendar or Muslim calendar, dating system used in the Islamic world for religious purposes. (Most countries now use the Gregorian calendar for civil purposes.) It is based on a year of 12 months: Muḥarram, Ṣafar, Rabīʿ al-Awwal, Rabīʿ al-Thānī, Jumādā al-Awwal, Jumādā al-Thānī, Rajab, Shaʿbān, Ramaḍān (the month of fasting), Shawwāl, Dhū al-Qaʿdah, and Dhū al-Ḥijjah. Each month begins approximately at the time of the new moon. The months are alternately 30 and 29 days long except for the 12th, Dhū al-Ḥijjah, the length of which is varied in a 30-year cycle intended to keep the calendar in step with the true phases of the moon. In 11 years of this cycle, Dhū al-Ḥijjah has 30 days, and in the other 19 years it has 29. Thus, the year has either 354 or 355 days. No other leap days or months are intercalated, so that the named months do not remain in the same seasons but retrogress through the entire solar...

  The introduction of the Gregorian calendar, depicted in relief on the tomb of Pope Gregory XIII in St. Peter's Basilica, Vatican City. Gregorian calendar, also called New Style calendar, solar dating system now in general use. It was proclaimed in 1582 by Pope Gregory XIII as a reform of the Julian calendar. By the Julian reckoning, the solar year comprised 365 1/4 days, and the intercalation of a “leap day” every four years was intended to maintain correspondence between the calendar and the seasons. A slight inaccuracy in the measurement (the solar year comprising more precisely 365 days, 5 hours, 48 minutes, 45.25 seconds) caused the calendar dates of the seasons to regress almost one day per century. Although this regression had amounted to 14 days by Pope Gregory’s time, he based his reform on restoration of the vernal equinox, then falling on March 11, to March 21, the date it occurred in 325 CE, which was the time of the First Council of Nicaea, and not the date of the equ...