@Alex893 These are the large group booking times in Chicago I can think of off the top of my head. You need to adjust your pricing for these days now. My son and daughter are already looking for places for 12-16 people for next years Lollapalooza figuring they'll get a deal booking so early like they did this year. Poor AirBnB owner had no idea Lollapalooza was during the time my son booked about 8 months out.
2018
September 27th - September 30th Chicago Expo
September 28th - September 30th Oktoberfest Chicago
Halloween Weekend and Halloween
Thanksgiving Week
Christmas Week
New Years Eve Week
2019
February 9th - 18th Auto Show
March 17th St. Patricks Day
Easter Week
May 24th - May 27th Memorial Day Weekend
July 11th - July 16th Taste of Chicago
August no date set yet usually first Saturday of the month Venetian Night
August 1st - 4th Lollapalooza
August 17th - 18th Air and Water Show
August 30th - September 2nd Labor Day Weekend and The Jazz Festival