When are you supposed to

Ours is the night before. Sometimes the rehearsal dinner is also thought of as the 'groom's party' in that the groom's parents often organize and pay for it (whereas the brides parents more often throw the wedding). My FH's family is huge and he has a bunch of OOT guests so we're having an 80+ person rehearsal dinner! We've been to a bunch of RDs the night before a wedding and I find them to be really fun warms ups to the big day.

ETA: Obviously often times the parents aren't involved in paying for any aspects of the wedding but if they are involved, I find it's pretty common to see the split be grooms parents doing the RD and bride's parents doing the wedding.