Re: How crowded is Magic Mountain during the week of Thanksgiving?
^ The park is actually open Wed/Thu/Fri of Thanksgiving week (as well as the weekend of course).
As for how busy it will be, not sure. I don't think I've ever gone that week, but November-February is generally a pretty dead time at the park. Even the Toys for Tots weekends at the park don't usually get very busy.
We went during the week between Xmas and New Years last year and the park was DEAD. I'd imagine the same would be true for Thanksgiving.