Fuck Breck go to some place off the beaten path it's tourist hell at that time.
Definitely January. The end of the December is the absolute worst. Tons of tourist traffic due to holiday trips without the great snow coverage and decent weather that usually accompanies spring break time.
You know i went this past Dec from the 22nd till Jan 1st and i must admit i didn't find it too busy to have a ton of fun. This may have to do with the fact that i live in Aus with small shitty crowded slopes but i really didn't find it too busy at all. I am sure if you are a local and are used to wide open runs and small crowds in general then of course xmas period to new years would be extremely busy, but i guess i just wanted to say my experience wasn't as bad as i was expecting.
The longest i waited in line was 25 mins and that was only because it was at the quicksilver lift at the bottom of peak 9 and i was with my 5yo daughter on her first lift ride. After that wait which seemed like forever we just lapped mercury, falcon. I found that peak 7 got really busy around 11am so we then went back to peak 9, 10. Didn't do a lot on peak 8 but found the CO super chair wasn't a long wait when we did ride 8.
The thing i would say though is, like linvillegorge said the snow coverage was shitty when i was there (about 50% open and not top of peak8) and when i go back (which i will) it will be end of feb start of march as i see its dumping there right now. As for Breck itself i loved the town and enjoyed how nice the locals were (didn't meet BA hahaha) but in all seriousness i loved my 10 days there and the people that i met really did make a great impression. Even my wife who is from Louisiana commented on how nice people are there and thats a lot coming from someone who prides themselves on southern hospitality.
my names's Petey and i have gigantic balls