How do I tell her I cannot make it?

Be

My friend asked me to go to a town day in the town I live in. Me and my fiancé have a graduation party but I said we’d probably be home after 5 based on others we attended this year(same family). Turns out that the party starts late at 5:30.

I feel bad but we cannot hang out with her. She’s meeting another friend of ours so won’t be a total loss for her.