Agreed, wait until no sooner than 8 weeks before thanksgiving or people will forget about it. Also, consider asking for a deposit for thanksgiving turkeys ($10-20 per bird) so you have committed buyers that have some skin in the game and will actually come and pick their bird up rather than letting a family member or friend convince them that a store bought turkey for .69c a pound is no different than buying locally raised.
So we raise broilers all summer and have weekly pick up days on our farm to pick them up (and now we also have weekly delivery days thanks to dang corvid). We do "advertise" the fact that we raise turkeys for thanksgiving pretty much all year, but we will not take commitments for them until mid-late September. I don't ask for a deposit personally because we have never had a problem with no-shows. But I also know where every single person lives that buys turkeys from me because I live in a very small town and only market to people that live near me. I have heard of farms in more dense locations or near cities that have had lots of problems with people not showing up on the pick up day.
Are you also raising the meat birds to sell for thanksgiving or for the whole season?