Date when you can celebrate the day you went behind the back of your ex-husband/wife/girlfriend/boyfriend and fell in love with your true love.
Guy: Hey sweetheart! Today is our Affairiversary, the day we started fooling around together and fell in love.
Girl: Aww.. it was the best day of my life.
Friend-to-guy: So dude, what are you getting your girlfriend for your affairiversary?