Well, does she want you to go with her for some form of family outing? It would be disrespectful to tell her you don't want to go, regardless of how long you've had the plans. You should have reminded her often that you were going surfing with your friends. If it's something your mom really wants to do or something that would make her happy, just do it as a good son. 9 out of 10 times, you're going to be hanging out with your friends, so if you're mom wants to spend time with you, don't take it for granted and try to bail.

Granted, if it's just some random errand she has to run, like, I don't know, printing some business cards at a shop in Santa Cruz (it's all I could think of), just politely ask if it would be okay if you did go surfing with your friends while you she is out doing whatever she needs to do.

You can go surfing any other time with your friends and can always reschedule. If they are your friends, they're not going to kill you for it. Just reschedule for next weekend (next weekend is obviously okay with your mom, since that's when she thought you were going anyway) so that everything is clear with your friends and your mom.

My parents seemed to take great joy in scheduling things at the same time I had plans with my friends, and I agree that it is really aggravating. I think it depends a lot on the frequency of spending time with your friends compared to spending time with your family. If it's a rare thing for you to spend time with your friends, then it might not be a bad idea to talk to your parents and suggest that you be let off the hook this time. If spending time with your friends is an every week thing, then it's a little bit unreasonable for you to expect your parents to bend their schedules around you every time. Conversely, if you rarely, if ever, have family outings, then you definitely should be going, especially if it's something of a big deal to your parents.

^ What she said is really good =]