We threw my grandparents a celebration for their 50th. We tried to keep it a secret, but they started to 1. plan their own and 2. wonder why no one seemed to care.

We had to tell them in the long run.
I see no problem having the party over Thanksgiving weekend. Families will already be together, maybe start mentioning it to some people and see what they think (the ones that will be attending that is!)
If it was me, I'd ask them about the vow renewal, and keep the reception a suprise. If they say yes to the renewal, I'm sure thier church might mention that "they are full" for all days they might want to do it, except for the day around Thanksgiving. Then afterwards, suprise for the reception....
I don't know, I'm just thinking of ideas out loud...
On a side note: Contact your State Rep via website to see how you can get congratulatory letters from them to help your parents celebrate their anniversary. My grandparents got one from the State Reps, senators, and the prez...