Julian calendar

How it works

A common Julian year has 365 days. Every fourth year February gains a 29th day, so that year has 366. The average Julian year is 365.25 days. Month names and lengths match the pattern later kept by the Gregorian civil calendar: January 31, February 28 or 29, March 31, and on through December 31. The machine is the leap rule, not the month labels.

That leap rule is a little too generous for the seasons. The year of the seasons is shorter than 365.25 days, so Julian dates drift later relative to the equinoxes. The Gregorian reform of 1582 tightened the rule: a century year is a leap year only if it is also divisible by 400. 1600 and 2000 leap on both calendars. 1700, 1800, and 1900 leap in Julian counting and are common years in Gregorian counting. Each skipped Gregorian leap day widened the gap.

In the present century the Julian date runs thirteen days behind the Gregorian date. Julian 1 January is Gregorian 14 January. If a church bulletin says “25 December (Julian),” the civil date in a Gregorian country is 7 January. The feast did not change its meaning. The numbering of the days did.

Julian leap year: every year divisible by 4, including century years. Gregorian leap year: divisible by 4, except century years not divisible by 400. Gap now: thirteen days, Julian behind Gregorian. Month lengths: the same 31, 30, and short February.

The Julian day used in astronomy is a different object with a similar name. It is a running count of days from a fixed epoch, not Caesar's month table. If a scientist writes a JD number, they are numbering a day for calculation, not dating a letter in the old civil calendar. Keep the two phrases apart.

Where the old table still shows

Julius Caesar's reform took effect in 45 BC after a long correction year. Pope Gregory XIII's reform took effect in October 1582 in the papal states: 4 October was followed by 15 October, dropping ten days to put the equinox back. Other states switched later. Britain and its colonies switched in 1752: Wednesday 2 September was followed by Thursday 14 September, dropping eleven days, because extra Julian leap days had accumulated. Russia switched in 1918. A family bible that spans a switch can look as if a week vanished. The weekdays were kept in order; the date numbers jumped.

Several Orthodox churches still reckon Pascha and some fixed feasts on the Julian table, then tell people the matching Gregorian civil date. That is why Orthodox Christmas is often kept on 7 January in Gregorian countries, and why Orthodox Easter can fall on a different Sunday from Western Easter. The Julian calendar in that use is a liturgical instrument, not a second bank calendar.

The Julian calendar did not lose the seasons overnight. It loaned them a quarter-day too many, every year, until the books had to skip. If you convert a historical date, name which calendar the writer used. “4 October 1582” in Rome is not the same kind of day as “4 October 1582” in a territory that had not yet switched. After a switch, the next date is not the next number. It is the next weekday with a new number.