Vendemiaire n. : The first month of the French republican calendar, dating from September 22, 1792.

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Taxonomy
décade, Vendémiaire, Quintidi, Décadi, Primidi, Duodi, Tridi, Quartidi, Sextidi, Septidi, Octidi, Nonidi, French Republican Calendar, the French, Romme, free encyclopedia, first month, Gregorian calendar.

Defination from the web:
Vendémiaire (F734). Vendémiaire was the first month in the ... Vendémiaire was the first month of the autumn quarter (mois d'automne) ... Like all FRC months Vendémiaire lasted 30 days and was divided into three 10-day weeks, called décades (decades) ...