December, with July coming in a close second.
Maybe I'm too conditioned to HS & college XC, but by December I'm ready to be done racing and training hard. The days get short, the holidays are upon us, the year is coming to an end and I want to think about other stuff. December is just a good time for things to end, the racing season included.
The good thing about December is a I live in the South, so it's still very easy to run because it's cool; but rarely really "cold".
July is a close second to worst because I live in the South it is a punishing, brutal month to run. The temps and humidity soar, and I'm still far enough out from Fall racing that I tend to question if it's worth the effort and daily dehydration.
The good thing about July is that it is the very beginning of training for the Fall. So you start setting goals, eye balling races and laying the ground work.