The annual celebration of a leap day marriage in any non-leap year. Usually celebrated on February 28 and/or March 1.
Wife: Since we got married on leap day, we don't have an anniversary this year.
Husband: Then I don't have to get you anything, right?
Wife: No, it's our unniversary. I still expect gifts.
An unniversary is the yearly celebration of the day you became divorced. Whether a happy or depressing celebration is up to the participant.
"Let's wish a merry unniversary to Ben, who is celebrating his sixth year since his soul-crushing divorce from Rebecca. How's that loneliness treating you?"