Well, let her thinkk aboutt it. Lol. If on that day she ends up saying NO. Then just say to her something like "really mom? If you knew the answer would b NO either way, then why'd you have me sitting hear thinking that your really thinking of it. Great, now I gotta tell my friends I can't go....even though I thought you were thinking about it, and at the end you would say yes, but I guessssss not" or something along those lines :) lol try to sound like "some1 feel sorry for me" [meaning you] lol
Tell your friends truth. Know won't be easy. Next time don't say yes till you heard yes. Make things easier for you. And maybe she'll say yes. Even explain to Mom you told friends yes thinking she would say yes. Consider lesson learned.
When your friends ask you to do stuff keep this phrase handy - "I'd love to do that, but let me check with my parents first to make sure it's ok." Yes, it stinks to have to get permission, but at least that way you aren't stuck in the position of having to go back on your word.
The best thing is to be honest with both sides by telling them the truth about both situations so as not to look like a coward at the end of the day. If it is something important, try convincing your Mom and let her understand the importance of the occasion.