Well I can’t say I have encountered an interesting local custom. There is word of something other worldly that resides in the northern portion of the state. The great spotted Hodag. I don’t think that qualifies as a custom, unless you count looking for it.
So if it is ok with you, I will share two football related customs I have been a part of.
In high school, we played in one of the oldest football rivalries in the state of Illinois. The “Old Oaken Bucket” football rivalry between Hinsdale Central and Downers Grove North began in 1935 as an annual contest between the two West Suburban Conference charter schools. The trophy, a copper-bound wooden bucket, was created by students in the [Hinsdale High School manual training room] to symbolize their rivalry, with results marked on it annually.

Our senior year, 1976, we won it back on Homecoming,, on Augie’s amazing touchdown. He is on the left. I am pointing out the score, 21-19.
Then in college, I got to play in one of the oldest rivalreis in college football. The Knox-Monmouth football rivalry began in 1888, ranking as one of the oldest in college football. Known as the “Battle for the Bronze Turkey,” it officially became a trophy game in 1928, proposed by Knox player Bill Collins because the annual matchup was held on Thanksgiving Day.

After getting spanked in that game my freshman and sophomore season, we took it back my junior year in 1979, and held onto it my last season as a football player. Here, Tom, Bobby and I formally receive the Turkey.
Those games, regardless of the outcome of the season, remain a focal point for each of those schools. And let me tell you, they are true battles where just about anything goes. I think I still have some of those boo boos left.
Perhaps that makes them customs.