Well he THOUGHT your bday was Saturday and y'all had the wedding. So you can't be all upset that he had plans for Sunday already. He was going with you to the wedding Sat, which he thought was your bday. As far as his brain was working, he WAS going to be with you on Sat and who knows, perhaps he has/had something planned for Saturday.
If, in his brain, your bday was Sat, then Sunday was to be as planned, going to his friends house. So he wanted you to come with him (good, he wants to be with you!). Would you rather he say "Oh your bday is Sun not Sat? Well let M. drop my plans with my friends and be with you J. the two of us" ?? Making a guy choose between his girlfriend and his friends is not a good idea. Guys don't like ultimatums like that. If I were you, I'd put on my big girl pants and smile. It's not worth it to get all hot and bothered. Take off your panties so they don't bunch up. I bet your bf will get turned on if you tell him you're panti-less. :) HA!