First off, I'd be prepared for one parent to leave early with DS, especially if the other parent can get a ride. In the event he gets OT and cranky/crazy (both of mine would have at that age), it won't do anyone any good to keep him up any later if he does fall apart. So.... have an exit strategy.
If you're able to have a longer nap or a catnap, do take advantage of it. If you're able to have him stay at home with someone familiar instead, do keep that option open. But if he is OT when he gets home (likely but not guaranteed), be prepared for a restless night, a wakeup the next day that's too early for him (or too late!), and a cranky day the next day while he adjusts. Also be prepared for a late night the next bedtime if he's not yet back on schedule.
Hope it does go well and you're able to have a properly good time!