The French Revolutionary Calendar had 12 months.
Created during the French Revolution, the calendar replaced the traditional Gregorian system with a more secular and decimal-style design. Each month contained 30 days and was divided into three 10-day periods called décades. The remaining five days in a common year, or six in a leap year, were added at the end as complementary days.
The months had evocative names linked to seasonal or natural features, including Vendémiaire for the grape harvest, Brumaire for mist, and Thermidor for heat. The calendar began with Year I, dated from the founding of the French Republic in 1792, although the system was formally introduced in 1793.
A common mix-up is to count the complementary days as an extra month. They were not a thirteenth month: they were separate year-end days used to keep the calendar aligned with the solar year. Napoleon abolished the calendar for civil use from 1 January 1806, though it briefly returned during the Paris Commune in 1871.