The French Revolutionary Calendar (or Republican Calendar) was introduced in France on 24 November 1793 and abolished on 1 January 1806. It was used again briefly during under the Paris Commune in 1871.

A year consists of 365 or 366 days, divided into 12 months of 30 days each, followed by 5 or 6 additional days. The months were:

The year was not divided into weeks, instead each month was divided into three décades of 10 days, of which the final day was a day of rest. This was an attempt to de-Christianize the calendar, but it was an unpopular move, because now there were 9 work days between each day of rest, whereas the Gregorian Calendar had only 6 work days between each Sunday.

The ten days of each décade were called, respectively, Primidi, Duodi, Tridi, Quartidi, Quintidi, Sextidi, Septidi, Octidi, Nonidi, Decadi.

The 5 or 6 additional days followed the last day of Fructidor and were called:

Each year was supposed to start on autumnal equinox (around 22 September)

Years are counted since the establishment of the first French Republic on 22 September 1792. That day became 1 Vendemiaire of the year 1 of the Republic. (However, the Revolutionary Calendar was not introduced until 24 November 1793.)

Leap years were introduced to keep New Year's Day on autumnal equinox. But this turned out to be difficult to handle, because equinox is not completely simple to predict. Therefore a rule similar to the one used in the Gregorian Calendar (including a 4000 year rule. It was to take effect in the year 20. However, the Revolutionary Calendar was abolished in the year 14, making this new rule irrelevant.

The following years were leap years: 3, 7, and 11. The years 15 and 20 should have been leap years, after which every 4th year (except every 100th year etc. etc.) should have been a leap year.

[The historicity of these leap year rules has been disputed. One source mentions that the calendar used a rule which would give 31 leap years in every 128 year period. Additional information is very welcome.]

The following table lists the Gregorian date on which each year of the Republic started: