The day in which a couple (or one member of a couple) achieves their first orgasm together. This is typically during coitus or foreplay. Like the "bangday," a couple's orgasiversary may be celebrated monthly or annually. If each member's orgasm was achieved on a separate day, the couple may choose to celebrate two orgasiversaries, or to celebrate the day when they both achieved orgasm during the same encounter.
Last week was our bangday, but next week is our orgasiversary, so we're going to celebrate next week instead.