1 definition by damit187
A day where it is acceptable for your significant other to fuck their "best friend." This person can be anyone of their choice but is often an ex girlfriend or boyfriend. It may be played off as "just hanging out" but when they don't come home that night you know what happened.
I know it's your first day off but I am going to go over to Trevor's for best friend day. Ignore why Matt is texting me.
by damit187 July 29, 2017