in laws taking over babies first birthday?!

I'm currently at my future in law's house and fiancé's father mentioned that he'd taken a holiday for our 3 month old son's birthday next September.

Am I wrong for feeling annoyed about this? Our baby is only 3 months old and we haven't said anything about celebrating our babies birthday (I mean he is only three months old). I don't know where we'll be or what our plans will be but now I feel pressured to be around when it's my son's birthday so they can be there.

My son's birthday is 6 days after my mother's so I was thinking of going to where my mum lives so we could have a joint celebration but even that isn't confirmed...nothing is. I just feel a bit backed into a corner. Am I wrong for this?