The autumnal equinox comes at MJD 56923.10331 (Tuesday, 2014-09-23 02:28:46 UT) in the Northern Hemisphere. Because that is 366 days after the previous one, the day before, 56922 (this Monday, Sept. 22), is a leap day in the French Republican Calendar. There are usually five complementary days at the end of the Republican year, sometimes collectively called Sansculottides, but this year there is a sixth complementary day, which is why the Republican leap year is called sextile. This day was called Fête de la révolution (Festival of the Revolution), commemorating the French Revolution.
Some people convert modern French Republican Calendar dates by using a rule like the Gregorian calendar, which usually has leap years every four years. This makes it easier to calculate, and is probably what the French would have eventually done, had they kept their calendar, but they did not. By this rule, 56922 (Mon., Sept. 22) is not leap day but actually New Year's Day, the first day of the month of Vendémiaire in the 223rd year of the Republic.
Leap day or New Year's Day, either way it's cause to celebrate!
Quartidi 4e jour complémentaire an CCXXII à 2 heures 95 minutes décimales t.m.P.