A large element of the French revolution was the secularization of society. Religion was seen as irrational superstition. One of the innovations after the revolution was the creation of a new calendar with DAY 1 being September 22, 1792. Since the old calendar revolved around religion on Sundays the new calendar changed the weeks to have 10 day weeks which were called decades. The calendar also eliminated all religious holidays like Christmas, Easter and Lent. The new holidays were in commemoration of the Revolution. The calendar actually was in use all the way up until 1805.
(The Great Courses – Living the French Revolution and the Age of Napoleon)