Of course it would time that way. You have had the plans for a year. If it would be hard to delay, if you would lose your downpayments. Is there some reason they cannot change the day or have a small celebration with you later or earlier. they are just going to have to understand.
We didn't have my son's birthday party on his birthday. I actually had it on a Saturday instead of the traditional Sunday this year and will continue to do that because his best friend would get a late start driving 3 1/2 hours home. I decided there was no reason I couldn't be a little more mindful of her schedule. She has never complained over the years.
Oh good luck. I hope they are understanding. Tell them the truth. Maybe they will help you arrive at a solution