If you haven't booked it already, I'd do it next year instead like you've already said.
However, I think it's a good chance to talk with them about expectations and taking you for granted.
I live close to my parents and MIL but I didn't assume any help from them. It was great that my mum would pop over with roast dinners for us. She'd also come in, wave at me while I breastfed in the living room and head straight to the kitchen to do some washing up, pack a back of laundry to wash and iron at hers, bring me a cup of tea and let herself out. She didn't have any help at all when I was born because we lived abroad so she just did what she missed.
My view was that the new baby and the birth is the responsibility of my husband and me as we're the ones that got us into this! We did ask if my mum would have our older son for a sleep over and our dog as we were having a homebirth and it made things logistically easier. But she was happy to help. As well as being an actual genuine help.
All that said, if she and my dad had a big holiday booked that happened to clash with the birth, I wouldn't have thrown my toys out of the pram. It's their life too! I'd have been fuming if my husband had booked to go to a festival that month but I had no expectation that anyone else put their lives on hold.
When my siblings were born we lived about 250 miles from our grandparents so one or another set would come to stay at our house when a new baby appeared, mostly to make sure the younger ones still had routine, went to school, brownies, had meals and all that. If you're staying in a hotel when you're there, what's happening with your other grandchild? Are they supposed to be with you at the hotel or?
Like I said, there needs to be a big chat about what's most useful, what their expectations are, and what you're able to give them support with. No harm in saying "I was about to book to go to (insert name of festival) and a trip to (insert name of country) but I'll do that next year instead".
I was birthing partner for my single bestie a couple of years ago and I overheard her a few months later saying to another friend that she'd not realised that asking me to do that meant I'd have to put a hold on doing anything else for the whole month just in case baby was a bit early. She was being lovely and was very grateful, she was just reflecting that she'd not clocked what a big commitment she was asking of me to not go on any holidays in my kids' summer hols and I didn't drink for the month as I wanted to be able to drive to her at any point just in case. I'm glad I did it and knew what I was signing up for, but sometimes the person asking for the help doesn't know fully the impact that it has on you unless you tell them.
Sorry for the essay!