We've scanned every flight from San Francisco to Chicago to find the best deals. If you want a cheap flight, it's best to be as flexible as you can on dates. If that day's no good for you, try searching by month to see some alternative dates.
Chicago Midway Airport is just one of the airports in Chicago. If you're not set on a specific airport, try changing your search from Chicago Midway Airport to Chicago to see a broader range of flights – they might be cheaper.
If you're planning on going to Chicago during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.