What should I do when my friends bail on me without apology?

Sounds like they just wanted to go to the party. You cant force people to go along with your plans and it sounds like they just didnt want to be obligated. They should have at least offered to reschedule so everybody could do what they wanted and everybodys happy. By bugging for an apology at this point you will seem like a big crab and they will probably talk crap about you being that way. Its called social grace. Sometimes people just go their own way, and you have to accept that or else people will see you as a hard case and not want to hang with you at all. If you're really that mad about it, then ditch them and find friends that want to do the stuff youwant to do.