The ancient lunar Roman calendar underwent many changes before the reforms of Julius Caesar which went into effect in 45 B.C. and changed it to a solar calendar. The Julian calendar consisted of the familiar twelve months and was 365 days in length.

The days of the months were not numbered sequentially as they are now. Rather three set points in the month which, in the traditional Roman calendar had originally corresponded to phases of the moon: Kalends (new moon), Nones (first-quarter moon), and Ides (full moon), were retained but reassigned --Kalends to the first day of the month, Ides to the middle of the month, corresponding to our 13th or 15th depending on the month, and Nones to the day nine days before Ides-- without regard to the moon. The remaining days of the month were denominated not, as in our system, by their relation to the beginning of the month, but by their relation to the next of those three fixed points /1/. For example, the Ides of March was the 15th; the 14th of March would be called PRIDIE ID. MAR., or "the day before the Ides of March." To further complicate things, the Romans counted inclusively, that is to say they counted the days between now and the next fixed point, plus that fixed point. Thus, the 13th of March was A. D. III ID. MAR., that is three days before the Ides of March, counting the 13th, 14th, and the 15th. The Fourth of July in the Roman calendar is A.D. IV NON. IUL., that is the fourth day before the Nones of July (in July the Nones being on the 7th). For days of the month after the Ides, the count looked forward to the Kalends of the following month. Thus December 25, in Roman usage would be A.D. VIII KAL. IAN., or eight days before the Kalends of January.

To keep the Julian calendar in sync with solar year of slightly less than 365-1/4 days, an extra day was added every fourth year to the month of February. Those bissextile /2/ years are known, colloquially, as "leap" years. The Julian calendar slightly over-adjusted for the difference between the true solar year and the 365-day year, and over the centuries required another adjustment to the bissextile years by Pope Gregory. The Gregorian calendar /3/ we now use retained the quadrennial bissextile year except in years evenly divisible by 100, in which case the year is bissextile only if it is also evenly divisible by 400.

JULIAN/GREGORIAN CALENDAR

/1/ The adoption of the current custom of numbering the days of the month sequentially, and the attendant disuse of the old system of designating the days by their relation to Kalends, Nones, and Ides, came about gradually. In England, for example, both the First Prayerbook of Edward VI (1549) and the Second Prayerbook of Edward VI (1552) as well as the Authorized Version of the Scriptures (1611) contain calendars that use both systems side-by-side. Viewers of Shakespeare's play Julius Caesar (ca. 1599) would have been quite at home with his references to the Ides of March.

/2/ Our calendars which number the days of the month sequentially from the first day of the month result in a 29th day added to the end of February in the "leap" year, however this is not strictly correct. The "leap" day is in fact, added before the 24th. The 24th day of February is in roman usage ante diem sextum calendas martii (A.D. VI KAL. MAR.), the sixth day before the Kalends of March, every fourth year the Julian calendar adds a second February 24, or bis, meaning twice, sextum, meaning sixth, day before March. In a bissextile year, the calendar for February looks like this:

MENSIS FEBRUARIUS Kalendae Februariae a.d.IV.Non.Feb. a.d.III.Non.Feb. pridie Non. Feb. Nonae Februariae a.d.VIII.Id.Feb. a.d.VII.Id.Feb. a.d.VI.Id.Feb a.d.V.Id.Feb. a.d.IV.Id.Feb. a.d.III.Id.Feb. pridie Id. Feb. Idus Februariae a.d.XVI.Kal.Mar. a.d.XV.Kal.Mar. Lupercalia a.d.XIV.Kal.Mar. a.d.XIII.Kal.Mar. a.d.XII.Kal.Mar. a.d.XI.Kal.Mar. a.d.X.Kal.Mar. a.d.IX.Kal.Mar. a.d.VIII.Kal.Mar. a.d.VII.Kal.Mar. Terminalia Bis a.d.VI.Kal.Mar. a.d.VI.Kal.Mar. a.d.V.Kal.Mar. a.d.IV.Kal.Mar. a.d.III.Kal.Mar. pridie Kal.Mar.

/3/ By the time Catholic Europe switched to the Gregorian Calendar in 1582, the Julian Calendar was off by ten days; when Great Britain adopted the Gregorian Calendar in 1752 the difference was 11 days. The last Europe nation to make the switch was Greece, in 1923.