Jan and Feb are the cool months, as well as typically the lowest months when it comes to attendance other than the days that happen to have a holiday. School is in and other than the New Year, is relatively nowhere near any peak seasons.
We went the week after Thanksgiving this last year! Did 4 days at Universal. The only busy day we had was on a Friday night. We saw lots of school groups that came in that night. Friday morning was fine.
ETA: Weather was very cool. Mid 60’s. We ended up having to purchase hoodies.
I've been in Sept, Oct, Nov, Dec, Jan, Feb, and May. I would say my best trips were when we hit weekdays that schools were in during Dec and Feb, and some Jan, though there seemed to be more foreigners in Jan driving crowds up. As long as you stay well away from any holiday dates you will be good, especially on Tuesday through Thursdays.