Bars are for guys and Bats are for girls.
Seventh grade was the main year for Bar Mitzvahs, since that was the year when most of us turned 13, the blessed age when one has the celebration. But let's get serious. The party was either during the day or at night. The day parties were not as good because no one likes to get crazy during the day. Plus, they were usually the cheap ones held in the 'party room' of the temple or, even worse, in the kid's own home.