If you are planning to use a fellowship hall and/or cater part or all of the party, start making those calls NOW. Why, you ask? Because when June hits, everyone else will realize that they also need to use that hall or book a caterer, and suddenly every caterer, deli, bakery, et al is busy, and the fellowship hall is booked solid. First come first serve as it were. That, and you can use it as ammunition against your MIL's plans. "But MIL, we already booked the hall/contracted a caterer! Relax and enjoy the party! We'll be sure to let everyone know closer to the event when and where it's happening."
Another good reason to nail down a date/time is so you can let your guests know. DD's friends will likely be having parties of their own, so you'll want to know whose party you're accidentally overlapping with so who can/can't come... and to let relatives know, because even though a grad party is a cool and fun thing, there are other things happening around that time - vacations starting, so on. I do think MIL needs to butt out.
I am not a fan of "combo" events, mainly because my bday falls very close to Thanksgiving. For much of my formulative years, my birthday and Thanksgiving were celebrated at the same mass family event and I HATED IT. When I was 12 or 13 I finally convinced my parents to let us split the events and do something much more low-key for my birthday. Actually, the final result was that the big Tday dinner was also scrapped in favor of just nuclear family. Nowadays we've rolled the b-day and thanksgiving into one again, but this is so much calmer and less crazy that I don't mind so much now. (And since I'm older I can now insist that my 'cake' not be pumpkin pie!)