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.
I don't really remember when we became facebook official... so lets just celebrate our bangaversary.