In January, fireworks and Fantasmic will only be on Friday, Saturday and Sunday if that matters to you. Fantasmic might potentially not be showing though, even on the weekends. Hours likely to be shorter on the weekdays, too.
Weekdays can be surprisingly crowded, you just never know. School in, school out, no one seems to care anymore.
1/11 and 1/12 is actually not the first weekend in January Magic Keys can go. Three of four Magic Keys are valid Sunday 1/5. First weekend after the holidays are over could be slower. Or not. It’s just so hard to know.
We went today 11/17, which is the first weekend of the holiday season. In theory it should have been nuts and I was expecting it to be slammed but it was fine. Crowded yes, but totally manageable. Parking was very slammed in the morning but that was my fault for showing up at 9:30 with the rest of the world.
We were in the park 10am to 7:30pm, used Multipass/LL/whatever we are calling it now and we rode 11 attractions, plus the train, plus had sit down brunch at Lamplight Lounge and it was a great day.