There are so many car events these days that inevitably one event will fall on--or be a week away from--another event. People will just have to choose which event they'd rather do and understand that there's no way to set a perfect event on a perfect date. Trying to plan around other events will only lead to complete and utter frustration.
I think earlier in May would be best. You can have rain ANYTIME, and while the possibility for cooler weather in MI still exists at that point the odds are in favor of warmer weather, and the track events in theory should be easier to schedule. Getting close to Memorial weekend would be a disaster, never mind holding it that weekend. Memorial weekend is good for a single-day event, but increased travel + family oriented holiday + the amount of single day events that take place would leave SZ hurting. Even holding it the weekend before may be a bad idea; I see people faced with a bunch of driving and a spent weekend for SZ only to turn around and have the same thing for the following weekend. May be fun for some, but for those with fairly busy schedules it could very quickly become overkill.
The 1st or 2nd weekend in May is generally a nice time of year in MI. This year was odd, what with the below average temps and above average rain, but coming from someone who's lived in MI for pretty much all his life, I don't think weather for early May should be a concern. Hell, we did SZ 01 and 02 in freakin' April and had great weather.
My .02.
91 SHO plus black/black, work in progress