sexiversary 

The same as an anniversary, but for the first time you ever have sex or fo the first time you ever have sex with the person you are currently dating, married to, etc.
(if your first relationship is when you first had sex, then it would be a double entendre)

1. the 12th this month is my sexiversary for the first time I ever fucked someone, but the 10th of next month is my (girlfriend's/boyfriend's/spouse's) and my sexiversary for the first time we've ever had sex.
2. yesterday was my (girlfriend's/boyfriend's) and my sexiversary.

sexiversary by obamapproved September 11, 2009

Sadaversary 

Its when you have a VERY sad occasion in ones life and it IS-NOT a anniversary, like the death of your wife or kids.

The sadaversary of my house buring down, next week is coming up.

Sadaversary by ALMILLER May 25, 2011

Separversary 

The annual date of separation from a relationship or job.

She celebrated her separversary from her toxic relationship.

Separversary by MsMonarch January 9, 2023