Alsace adopted the French Republican calendar during its use from late 1793 through 1805, after which they reverted to using the Gregorian calendar - the one you are familiar with and use today. In an effort to do away with the past and usher in the modern era, French revolutionaries changed many standard systems, including measurement, their monetary system, and the calendar. They sought to simultaneously decimalize the calendar and diminish the references to monarchy and religion which had become inseparable from the old calendar. Instead of Saint's days, the days were connected to farm life and named for plants, animals, and tools. Instead of months named for monarchs, such as August and July, or for gods, like January and March, the months were named for weather conditions and harvests. They restructured the year to be composed of twelve months beginning at the autumn equinox. Each month was made up of three ten-day weeks called "decades". At the end of the year five complementary days were added to keep the calendar stable with the solar year. On leap years a sixth was added, and called "Revolution Day".
The calendar began on September 22, 23, or 24, depending on when the equinox was reached in a particular year, and the months are as follows: Vendémiaire (grape harvest), Brumaire (fog), Frimaire (frost), Nivôse (snowy), Pluviôse (rainy), Ventôse (windy), Germinal (germination), Floréal (flower), Prairial (pasture), Messidor (harvest), Thermidor (summer heat), and Fructidor (fruit). They also removed reference to the birth of Christ and restarted their calendar at the year one. All years were represented by capital Roman numerals. The days of the week were assigned numbers, like primidi, duodi, tridi etc. all the way to décadi. Thus a date for a birth might look like this: quintidi Thermidor VII, or 16 Ventôse IX.
The note on the left side of the above birth record reads "Joseph Schweitzer / 17th frimarius X / Jahr Republik," or Joseph Schweitzer, the 17th of Frimaire in the tenth year of the Republic.
If you are researching a family whose dates run through 1793-1805, you will find that the civil records like this one will all bear dates from the French Republican calendar. To avoid confusion, most online family tree makers will only accept dates in the Gregorian format, so you will need to use a conversion tool or table to discover the corresponding day. I like the converter at Napoleon.org. The format is straightforward and easy to use, and can be found at
http://www.napoleon.org:81/en/essential_napoleon/calendar/index.asp
