Source : Webster's Revised Unabridged Dictionary (1913)

Vend'emiaire \Ven`d['e]`miaire"\, n. [F., fr. L. vindemia
   vintage.]
   The first month of the French republican calendar, dating
   from September 22, 1792.

   Note: This calendar was substituted for the ordinary
         calendar, dating from the Christian era, by a decree of
         the National Convention in 1793. The 22d of September,
         1792, which had been fixed upon as the day of the
         foundation of the republic, was also the date of the
         new calendar. In this calendar, the year, which began
         at midnight of the day of the autumnal equinox, was
         divided into twelve months of thirty days, with five
         additional days for festivals, and every fourth year
         six. Each month was divided into three decades of ten
         days each, the week being abolished. The names of the
         months in their order were, Vend['e]miaire, Brumaire,
         Frimaire Nivose, Pluviose, Ventose, Germinal,
         Flor['e]al, Prairial, Messidor, Thermidor (sometimes
         called Fervidor), and Fructidor. This calendar was
         abolished December 31, 1805, and the ordinary one
         restored January 1, 1806.