What should I do?!

My partner's birthday and my friend's wedding falls on the same day.

I'm only invited to the after party, which is 7pm-11pm.

My partner never really has good birthdays. Something always goes wrong unfortunately so I like to spend the day with him to make him happy and do whatever he pleases, which is why I'm worried about going to the wedding.

But I'm really good friends with the groom and he has always been very supportive of me when going through rough stuff at work (I work with him). And in my mind I'm thinking 'weddings only come around once in a lifetime'. I think he would be disappointed if I couldn't come.

BUT I'd feel bad if I ditched my partner in the evening to go to a wedding on his birthday. Or if my partner can come as a +1, I'd feel bad that I'm dragging him out to a wedding on his birthday and making him dress up!

What do I do!?