Midversary 

The six month celebration of a relationship,which can only occur once before the first anniversary.

Person A met Person G on the 17th of May therefore their midversary would be on the 17th of November.

Midversary by CousinRosiex October 10, 2013

miniversary

Like an anniversary, a date marking the successful passage of time in a relationship.

They've been going out one week and already have celebrated their first miniversary.

miniversary by P-fizzy December 14, 2005