The French Republican Calendar (French: calendrier républicain français), also commonly called the French Revolutionary Calendar (calendrier révolutionnaire français), was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871. The revolutionary system was designed in part to remove all religious and royalist influences from the calendar, and was part of a larger attempt at decimalisation in France (which also included decimal time of day, decimalisation of currency, and metrication).
Sylvain Maréchal, prominent anticlerical atheist, published the first edition of his Almanach des Honnêtes-gens (Almanac of Honest People) in 1788. On pages 14–15 appears a calendar, consisting of twelve months. The first month is "Mars, ou Princeps" (March, or First), the last month is "Février, ou Duodécembre" (February, or Twelfth). (The months of September [meaning "the seventh"] through December [meaning "the tenth"] are already numeric names, although their meanings do not match their positions in either the Julian or the Gregorian calendar since the Romans changed the first month of a year from March to January.) The lengths of the months are the same; however, the 10th, 20th, and 30th are singled out of each month as the end of a décade (group of ten). Individual days were assigned, instead of to the traditional saints, to people noteworthy for mostly secular achievements; December 25 is assigned to both Jesus and Newton.
Later editions of the almanac would switch to the Republican Calendar.
The days of the French Revolution and Republic saw many efforts to sweep away various trappings of the ancien régime (the old feudal monarchy); some of these were more successful than others. The new Republican government sought to institute, among other reforms, a new social and legal system, a new system of weights and measures (which became the metric system), and a new calendar. Amid nostalgia for the ancient Roman Republic, the theories of the Enlightenment were at their peak, and the devisers of the new systems looked to nature for their inspiration. Natural constants, multiples of ten, and Latin as well as Old Greek derivations formed the fundamental blocks from which the new systems were built.