Took me a sec to realize why 8/28 seemed like a significant day. Then I thought oh yeah, Plainfield tornado. 32 years.
Plainfield and Oak Lawn are basically 1a and 1b for me as far as historical tornadoes in Chi metro, because of the amount of death and destruction caused by each. Everything else is in tiers below that with the possible exception of the 3/28/1920 tornado that went through western/northern Cook county.
Even though Plainfield was F5, I don't know if I'd call it "worse" than Oak Lawn. That F5 was based on damage to a field, not to any of the buildings, so it's quite likely that the same tornado today would not be rated over EF4. I think the Oak Lawn F4 rating would probably hold up as EF4 though as some of the damage in Oak Lawn looked pretty extreme, at least based on pics. Plainfield does have the out of season claim though, as it's extremely rare to get something of that magnitude at this time of year (especially in IL).