Thirty Days Hath September Meton in the 5thcentury. Rather than adopt a new system like the Egyptian calendar, which had 12 months of 30 days each and a set, annual intercalary month of 5 days, Caesar aimed for his 46 reform to maintain as much continuity as possible with the old calendar. Ultimately, Mercedonius was removed, the four existing 31-day months were maintained, February was left unchanged apart from leap years, and the needful additional ten days of the year were added to the 29-day months to make them either 30 or 31 days long. By the Renaissance, the irregularity of the resulting system
Leap year be kept holy and the feast days and Sabbaths. Years consisting of 12 months have between 353 and 355 days. In a ("in order") 354-day year, months have alternating 30 and 29 day lengths. In a ("lacking") year, the month of Kislev is reduced to 29 days. In a ("filled") year, the month of Marcheshvan is increased to 30 days. 13-month years follow the same pattern, with the addition of the 30-day Adar Alef, giving them between 383 and 385 days. The observed and calculated versions of the Islamic calendar do not have regular leap days, even though both have