The 6th monthiversary. Coined from semester and anniversary. Sometimes counting each monthiversary get too much but your anniversary is still a while away, so you count every 6 months.
A: "Happy 90th monthiversary!"
B: "Happy 15th semestriversary!"
*lots of smooching out*
C: "Isn't it time you guys quit it and just counted years? and got a room...seriously."