If you made reservations for lodging, travel, furnishings or anything else, you'll want to check them one ore two weeks before the reunion. That way, the companies will know you're coming and if there's been a mistake you'll have time to take care of it. It's not unheard of for a hotel or airline to lose reservations. You should have confirmation numbers if you made reservations online.
The ideal time to check reservations is just before you send out reminder invitations. That way, if something needs to be changed you can mention it on the reminders.