In its most primitive form the Roman calendar apparently had 10 months, which were (to use corresponding English terms whenever possible): March (31 days), April (29 days), May (31 days), June (29 days), Quintilis (31 days), Sextilis (29 days), September (29 days), October (31 days), November (29 days), and December (29 days). To fill out the 365 days a number of blank days or occasional intercalary months were used. Later, January (29 days) and February (28 days) were added at the end of the year.
In the time of the early republic the so-called year of Numa was added. The Romans thus arrived at a cycle of four years: the first year and the third year had four months of 31 days, seven of 29, and one, February, of 28; the second year had a February of 23 days and an intercalary month of 27 days; the fourth year had a February of 24 days and an intercalary month. The chief trouble with this system was that in a four-year cycle there were four days too many. What was worse, the pontifex maximus was given the power soon after 200 B.C. to regulate the calendar, and the practice grew of using the intercalations for the promotion of political ends to lengthen or to shorten an official's term.
When Julius Caesar became pontifex maximus, the Roman calendar had been so much abused that January was falling in autumn. At this point the methods of the Egyptian calendar were borrowed for the Roman. Julius Caesar, on the advice of the astronomer Sosigenes, added 90 days to the year 46 B.C. (67 days between November and December, 23 at the end of February). This caused the spring of 45 B.C. to begin in March. To retain this position of the seasons, he changed the length of most of the months: March, May, Quintilis (later named July after Julius Caesar), and October he left as they were; he added 2 days each to January and Sextilis (later named August to honor the Emperor Augustus); February was 28 days long except that in every fourth year a day was inserted between the 23d and the 24th of the month.
In Roman computation three days in the month were used for counting the date. These three were the Kalends (1st day of the month), the Nones (the 7th day in March, May, July, and October, the 5th in the other months), and the Ides (the 15th day in March, May, July, and October, the 13th in the other months). The days were counted before, not after, the Kalends, Nones, and Ides. Thus, Jan. 10 was the fourth day before the Ides of January or the fourth day of the Ides of January, because the Romans counted inclusively. Jan. 25 was the eighth of the Kalends of February, Feb. 3 was the third of the Nones of February. Feb. 23 was the seventh of the Kalends of March and remained so when an intercalary day was inserted every fourth year between it and Feb. 24; hence in a leap year there were two days counted as the sixth of the Kalends of March. The leap year was therefore called bissextile [Lat., = sixth twice]. There is a legend that alterations in the length of the months were made later by Augustus to flatter his own vanity, but there seems to be no foundation for this story.
The Julian year is 365 days 6 hr, hence a little too long. Therefore, by the 16th cent. the accumulation of surplus time had displaced the vernal equinox to Mar. 11 from Mar. 21, the date set in the 4th cent. In 1582 Pope Gregory XIII rectified this error. He suppressed 10 days in the year 1582 and ordained that thereafter the years ending in hundreds should not be leap years unless they were divisible by 400. The year 1600 was a leap year under both systems, but 1700, 1800, and 1900 were leap years only in the unreformed calendar. The reform was accepted, immediately in most Roman Catholic countries, more gradually in Protestant countries, and in the Eastern Church the Julian calendar was retained into the 20th cent. The present generally accepted calendar is therefore called Gregorian, though it is only a slight modification of the Julian.
The reform was not accepted in England and the British colonies in America until 1752. By that date the English calendar was 11 days different from that of continental Europe. For the intervening period before the reform was introduced into the English calendar, the Gregorian style is called the New Style (N.S.), and the Julian the Old Style (O.S.). New Style years begin Jan. 1, but Old Style years began usually Mar. 25. Thus Washington's birthday, which is Feb. 22, 1732 (N.S.), was Feb. 11, 1731 (O.S.). To avoid confusion sometimes both styles are given; thus 11 Feb. 1731/22 Feb. 1732.