Cancer Season is the fourth season in the game. It started on December 1st, 2019 with a game update.
The drop places and rates of all parts and recipes changed. As of December 8th, it looks like parts and recipes drop at the same times, but appear in all quest types now, e.g. a part that could previously be found only in swamp can now drop in valley or forest, too.
For an overview of the new drops see List of T4 Drop Times (Cancer Season).