Do you think it would be alright if I didn't go to my best friend and housemates hen party?
Well why don't you want to go?
I have something to do that day
That's fine. If you have other plans, you have other plans.
Yes, I will be going out with other people that day and I am glad that you understand.
If you have other things to do already, then it is important to honor those first. I'm sure the people that you're hanging out with will appreciate you not cancelling on them.