The Tembarican Calendar
While it is both the most accurate and the original calendar of Omeria, the Tembarican calendar is rarely used by anyone but historians and mages. This is because it sacrifices ease of use for accuracy, tracking all phases of the moon and the full 1566 days it takes for Omeria to complete both clockwise and counter-clockwise orbits of all the three suns. The calendar year is divided into six cycles, depending on which sun is being orbited and what direction the orbit is moving in. The months last 232 days when the orbit is going clockwise, and 290 days when it's moving counterclockwise. With the moon taking 29 days to orbit Omeria once, this leads to cycles with respectively 8 and 10 months.
Worldanvil please make calendars better so that I can make the full one actually look good :(