I would definitely be out of town that day.
But I would also probably put up on my Facebook something like: "Just booked a hotel for the weekend out of town with the girls to celebrate the end of the school year. We are all looking forward to it."
I wouldn't say where or give details. But it lets friends and family know there's a clash if he's planning a party or something in advance. They can contact you if they have questions.
If he's not planning anything, it's harmless.
If people show up at your door unaware you are out of town believing there's a surprise party for you, you don't look bad because you gave advance notice on being gone, how would you know he was doing this fool thing? You're divorced for g sake. He can't make you look bad.
I wonder if he really is planning a party--that's a lot of work. i think he's collecting information. The whole thing is creepy and unnerving, no doubt.