I would definitely say stay away if it was in her home, but a public place makes it a little more optional. Since you are already having a party with your family then I would probably stay away also, just keep things easier. I have tried to stick with the plan that less interaction means less drama. I do think it was inconsiderate of your X to plan it on your day as it decreases the amount of time you get to spend with your daughter. I think your plan to inform your X that you will be picking D8 up after the party sounds good. I have also found that less options given mkes things easier, just state your plan and go with it.
"You can't stop the waves, but you can learn to surf." Jon Kabat-Zinn
