If the cat colony is hungry enough, they can be trapped any time of year. When I first started doing TNR: Trap-Neuter-Return, I was trapping year-round because kittens were being born year-round. I also was trying to get an injured cat. Now that the colony numbers are stabilized I can be more selective with the timing.
This was when I first trapped the Eleanor Rigby Colony a few years back. The outdoor cats were bonded and swarmed the traps together, completely ignoring the snow. They were very interested in the canned sardines in oil that I used for bait.