Of course, I don't mean girls literally hibernate like bears.
What I mean is what I found out running Game in Atlanta.
Folks couple up. More girls give you legitimate boyfriend excuses. More guys you know, and guys on Game forums, talk about LTRs.
College girls also gear up for exams, papers, projects, etc.
Older girls - and college girls with daddy's money - think about a winter vacation. Preferably, to a warmer place with desperate guys, such as Spain, Italy, or Turkey (all known for being sex tourists). And especially if the girls are not lucky or hard-working enough to have locked down a boyfriend.
Note that this also aligns with Game gurus such as Krauser posting when they too go into winter hibernation.
Of course, you can still get some girls during winter. Shoot, I almost SDL'd a Warsaw girl in Sopot near the end of November. (But let's face it, she was there alone with nothing to do - a lucky break for me.)
But, if you're planning a trip, or simply taking breaks from the Game, as Roosh advocates, and Krauser implies, then winter is a good time to stay out of the Game.
Research does indicate that breakups happen before Christmas and Valentine's Day. However, my guess is that these girls just sulk, eat chocolate or ice cream, and get fat until spring rolls around, when they can no longer hide their newly-gained rolls with winter clothing.