The French Republican Calendar went into effect on October 24, 1793. However it began in year II and back-dated the start of year I to September 22, 1793. Napoléon Bonaparte officially ended its use on January 1, 1806.
When Grimod de la Reynière was composing the first edition of the Almanach des Gourmands in the autumn of 1802, the Gregorian calendar’s days of the week had only months earlier (on Easter Sunday, April 18, 1802) come back into official use as a result of Napoléon Bonaparte’s 1801 Concordat with the Roman Catholic Church, which re-established it in France.
It is therefore significant that Grimod chose to organize his calendar according to the ancient Gregorian System, beginning with January. His mentions many Catholic feast days and holidays emphasized his inherent conservatism (or, rather, anti-Revolutionary zeal).
The 1803 Almanach des Gourmands did, however, include an optional conversion guide (see below) to the two systems, which was glued into some but not all copies. For example, the first edition of 1803 at the Schlesinger Library contains the calendar but the second edition of 1803 does not. In the copies of these same editions at the Bibliothèque nationale de France, the situation is reversed. It is the 2nd edition of 1803, which contains the guide to the Republican months, while this is absent from the 1st edition.