What is the busiest day?

I am wondering what the busiest days of the week are for an upscale casual dining restaurant in the summer. I get paid a percentage of sales so I want to request to work on typically busy days. I know that during most seasons it's the weekend, but I heard from a couple of people that in the summer it's the reverse. Any incite would be much appreciated. If it's any help I work in an upper-middle class suburb of Chicago.