The anniversary of when a couple first banged. Usually celebrated because that is how a couple met and initiated their relationship. The couple may be unclear of when they technically began "dating" so instead, they have to commemorate the first time they began banging. However, a bangaversary is not celebrated upon any two people having sexual intercourse. The initial bang can turn into one of either two scenarios; 1. a one night stand or 2. a steady sexual relationship that may turn into a boyfriend/girlfriend status, in which case a bangaversary is celebrated.
Gf: Happy Valentine's Day, baby! What should we do today?
Bf: Happy Valentine's Day to you too, babe. Well, today is also our blowversary, so I was thinking we could celebrate with a bj.