What should i say to my dad because i'm not going round tomorrow?

Dad took us all to a theme park today and me and my brother were mucking about - as usual, we're crazy, bubbily and hyper personalities. We're bound to be nutters. Anyway, dad takes it seriously. He's like 'YOU COME OVER HERE, YOU GO OVER THERE AND STOP EMBARRASSING ME!' Were his Kids. Like it or leave it. Were fine, people like us. It's not like were going to see anyone there ever again anyway. We werent even that bad. But my dad always has these bad moods. He's never loved me or my brother. He's always not wanted to know. It's only beacause he's got a new girlfriend who is nice to us, he feels he has to be nice to us. His old girlfriend said we were 'brain damaged' with 'mental problems' who needed 'help' Mainly because our mom's side of the family suffer from Asbergus and mentality problems. My dad took her side then. But something isn't right with my dad and mine's relationship. I don't feel comfortable. I go round to his on fridays, but i sometimes don't turn up for about a month. I stay at mom's because she's always loved me and she never goes into a mood and it's my home. Anyway, today, my dad turned out to be a total knob for other reasons. I'm not going round his tomorrrow. What do i say? I have an exam.. What do i say?