Antiversary

Antiversary
-noun

  1. Any day which is not an anniversary for the person or people referred to.
  2. A day which would have been an anniversary if things had turned out differently.

Derivation

  1. A punning backformation on anniversary, bearing the same relation to its original as unbirthday does to birthday
  2. A more cynical variant, leaning more heavily on the ‘anti’ prefix for its meaning.