Best way to ask my dad to let me go to my boyfriends Christmas party?

So my dad doesn't exaclty like my boyfriend. He thinks hes a nice guy and all but the doesn't like the fact we're dating. Because I'm "too young" I'm 16 and everyone tells him I'm not too young but whatever he's just being a dad. Anyway, my boyfriends dad is having a Christmas party and he invited me. His family is going to be there so theres no way anything bad/inapropriet will happen . He said we can hang out in groups and whatever so normally I'd figure he'd be okay with it but I just got done being grounded for a month so I'm a little scared to ask. Whats the best way to do it? I'm already helping around the house extra and I'm gunna ask when he's in a good mood, but what else? I really want to go