The calendar as it currently stands was an invention of the southern coalition before the empire was created. It was first used in the Estate of Alyssenne and Estate of Lyrienne and progressively extended through their allies until the whole south was sharing it, so as to make trade exchanges and military alliances easier to coordinate.
Meanwhile, the north did not have a common calendar system. This meant that when the two coalitions joined together into one empire, it was natural to adopt the southern calendar as the official imperial calendar. Most Estates adopted the technical details of the calendar but transposed their own named for months and days on top of it, in accord with their own seasons and events. Some Estates, however, have preferred to continue to use their own calendar system and to convert date to the imperial system only when strictly necessary. This, of course, results in a number of mistakes being made.
The year is made of 12 months of 30 days each, and each month is made of 5 weeks of 6 days. To complete the 365-day calendar, the year ends with a special 5-day week that does not belong to any month and is called "feast week", or "festival weeks".
The months are named after something notable about it: 1) Brumial, named after fog; 2) Nivial, named after snow; 3) Fleurial, named after flowers; 4) Frigial, named after cold in reference to hail; 5) Naissial, named after births; 6) Fruitial, named after fruits; 7) Solial, named after the sun; 8) Aurial, named after goldin reference to wheat; 9) Vendial, named after vendage, the grape harvest; 10) Feuillial, named after leaves; 11) Pluvial, named after rain; 12) Ilexial, named after holly.
Each day is named after its number: 1) primidi, 2) duodi, 3) tridi, 4) quatridi, 5) quintidi, and 6) sixtidi.
The most notable event of the year that are celebrated through the empire are the Solstices and Equinoxes, occurring on the 20th Fleurial, 22nd Fruitial, 24th Vendial, and 25th Ilexial.