Save the date for a milestone birthday party.

[SillyG]

Just been sent a WhatsApp message to save 2 dates in December as possible dates for an 18th birthday party.

Is this usual? Seems very late to be organising a milestone birthday and especially in December.

This! ^

You're being really weird about this.

Nobody is forcing you to go to the party. They've told you there's going to be a party and they're letting you know in advance. If you don't want to go, simply say 'Sorry, can't make either of those dates as we've got Christmas stuff already arranged'. It's really not complicated.

Even if it's the 27th, I will be exhausted

OK, but how is that the party organiser's fault? They've chosen a date for the party that suits the person whose birthday it is, and it's up to you whether you go or not. They can't pick a date based on whether you happen to be someone who is tired out by socialising two days in a row.

I’d just reply to say you can’t do Boxing Day as you already have plans, and let them know if you’d attend on the 27th if that’s the date they go with?

If it's someone close to you surely you know it's their big birthday coming up? All they are letting you know is they are going to have a celebration one of those dates. If you know now you can't make either let them know if you have nothing planned and want to go save the dates
Presumably they have suggested two days as they want to book something but want to rest the waters with guests first