Originally posted by puppyluvs
I would just say I am sorry I have to much to much personal business to take care of this weekend and I am not gonna be able to make it !
You are better off saying that because then you are less likely to get stuck in a lie. The more specific you are, the more likely to get caught.

Also, next time around, instead of lying...I'd say "Oh, shoot! I wish I'd known sooner. I can't make it, we already have plans."