Busy weeks are Thanksgiving week and the week between Christmas and New Years. The busy time sometimes extends until the weekend after New Years because often kids are still out of school.
The weeks between Thanksgiving week and Christmas are sparse with the week right before Christmas particularly deadly. Too much shopping and party-going.
President's Day week is also busy. Kids are out of school. Easter/Passover weeks ditto.
Weak times are January and early February, the 2-3 weeks post Labor Day, 4th of July if they play at all, Rosh Hashanah, Yom Kippur and Christmas Eve which is so bad that shows rarely play that night.
You can spot weak times because those are the periods that things are put on "mid-winter sale" like Phantom has done over the years and some other long runs without big advances for those times.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher