Excerpt taken from the book The Story of French (Ch. 6)
The revolutionary government hired the poet Fabre d’Églantine–better known for his bedtime song “Il pleut, il pleut, bergère” (“It’s raining, it’s raining, shepherdess”)–to come up with new names for the days and months. D’Églantine was inspired by the weather and natural cycles, so he used different suffixes for each season, attached to Latin words that corresponded to the typical weather for each month. The fall months were Vendémiaire, Brumaire and Frimaire; the winter months were Nivôse, Pluviôse and Ventôse; the spring months were Germinal, Floréal and Prairial; and the summer months were Messidor, Thermidor and Fructidor. D’Églantine wanted to rename the days after vegetables, animals and farm tools, but the National Assembly probably realized that they were already pushing their luck by trying to name the days after Latin numbers (primedi, duodi, tridi and so on).
This poetic approach had problems, the biggest being that the previous calendar worked (even if the revolutionaries didn’t like it) and everyone was used to it. In spite of its universal pretensions, the revolutionary calendar was based on the seasons around Paris, and it didn’t apply well to other areas of France such as the Alps. And the revolutionaries didn’t exactly make the calendar easy for people to use: Day 1 of Year I was September 22, 1792, the day the monarchy had officially ended. That meant that January 1 fell on II Nivôse. Robespierre, for instance, was ousted on 9 Thermidor An II (July 27, 1794)–which is why the episode is still known as Thermidor. The promoters also shot themselves in the foot by allowing only one day of rest in ten, rather than the one in seven of the old system. Napoleon abolished this impractical and unpopular calendar on II Nivôse An XIV–January 1, 1806. But French laws established while it was in use are still referred to by their Republican date.