Wikipedia
The Sansculottides (; also Epagomènes; French Sans-culottides, Sanculottides, jours complémentaires, jours épagomènes) are holidays following the last month of the year on the French Republican Calendar which was used following the French Revolution from approximately 1793 to 1805.
The Sansculottides, named after the Sansculottes, append the twelve, 30-day months of the Republican Calendar with five complementary days in a common year or six complementary days in a leap year so that the calendar year would approximately match the tropical year. They follow the last day of Fructidor, the last month of the year, and precede the first day of Vendémiaire.
The Sansculottides belong to the summer quarter. They begin on 17 or 18 September and approximately end on the autumn equinox, on 22 or 23 September on the Gregorian calendar.