The positivist calendar was a calendar reform proposal by Auguste Comte in 1849. Revising the earlier work of Marco Mastrofini, or an even earlier proposal by "Hirossa Ap-Iccim" (Hugh Jones), Comte developed a solar calendar with 13 months of 28 days, and an additional festival day commemorating the dead, totalling 365 days.
This extra day added to the last month was outside the days of the week cycle, and so the first of a month was always a Monday. On leap years, an additional festival day (also outside the week cycle), to celebrate holy women, would join the memorial day of the dead. The scheme followed the Gregorian calendar rules for determining which years are leap years, and started on January 1. Year 1 "of the Great Crisis" (i.e. the French Revolution) was equivalent to 1789 in the standard Gregorian system.
Much like Comte's other schemas, the positivist calendar never enjoyed widespread use.