It's actually not that bad. It's Christmas week. Spend the night, go out and celebrate the victory and head back on the 23rd. Enjoy the early Christmas present from Kylan and the Balkans.
I feel like this is how they always view this ... people who are going are likely taking the next day (and then the holiday) off work, and they want to avoid having it the day before Christmas Eve. If you go too far before the 20th, it loses its "pre-Christmas" festive feel. This is when it has occurred in recent years:
Sunday 12/22/2024 (18,497 attendance)
Friday 12/22/2023 (18,485 attendance)
Thursday 12/22/2022 (18,452 attendance)
Wednesday 12/22/2021 (14,953 attendance - literally during Mizzou's bowl game)
Saturday 12/21/2019 (15,259 attendance)
Saturday 12/22/2018 (16,397 attendance)
Saturday 12/23/2017 (21,289 attendance)
Wednesday 12/21/2016 (12,409 attendance - all-time low)
Wednesday 12/23/2015 (14,456 attendance)
Saturday 12/20/2014 (20,079 attendance)
So the game was played between December 21 and December 23 in 9 of those 10 years, with the only exception being 2014 on the 20th. It seems pretty clear they care more about it "kicking off the Christmas holiday" than the day of the week.
And while there does seem to be some correlation between Thursday-Sunday and higher attendance, I think it has more to do with how good each team is. You are pretty much guaranteed 9-10K Illini fans and 4-5K Mizzou fans every single year, no matter what. If Illinois is halfway decent, we will get to our 11K half of the tickets easily. If Mizzou is decent, they will sell 7K+ tickets and get us past 18K, and when both teams are good at the same time, you start getting the really big crowds.