Assclunch originated from the the latin phrase "A mari usque ad marius" which was a sexual act where the Emperor of Rome would two butt cheeks, one a man, one a woman and insert his penis between them both, effectively making himself an instant-bisexual.

The phenomenon of having to come out as bisexual to the same person/people repeatedly because they assumed or hoped that your sexuality was a "phase." Refers less to the holiday and more to the 1993 Bill Murray comedy film in which the main character experiences the same day over and over.