The five complementary days added to the twelve months of the Revolutionary Calendar. Each month being made to consist of thirty days, the riff-raff days which would not conform to the law were named in honour of the sans culottes, and made idle days or holidays.
sans-culottism. Red republicanism.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Sans Culottides from Fact Monster: