Traditionally, it starts when F1 starts racing. We try to fit two seasons in one F1 season span. We start "first season" early and finish "second" season late is the usual configuration.
Tracks and racedates follow closely to F1 two week pace but there are always compromises. At best you got F1 weekend two days after our race or 3 days before.
Exact Season 12 calendar is not even drafted yet. I'm sure the off season starts with a break, people assemble new rigs, upgrade PCs if needed. We most likely will do again some fun races testing new tracks for the next season. Plus i got one cold laps event planned (no pre-practice, new track, never played by anyone, password announced 30mins prior to event start) that will test everyones ability to drive in dangerous conditions: The Champions Race (possibly with classic F1 mod, changeable weather, track is designed to be wet/damp with interesting surface changes..).