You're a tool. Don't you think it was INCONSIDERATE of them to change the plans (which included OP) and then not let OP know???
It's actually considerate behaviour if you have plans with someone (ie, OP) and then change them, to inform them of that... duh. OPs friends were inconsiderate and rude to do what they did. Period. Then OP could either join them or make alternate plans, or in OPs case, go to work.
a) Tell them how i feel and go (knowing i'll still have fun, cause they're great friends)