(Using Supernatural logic here)
When Cain and Abel were born, Lucifer had already planned on taking Abel. After they made their sacrifices to God, Lucifer appeared to Cain. He told him how he planned to take his brother, but Cain would not let him. Lucifer said if Cain killed Abel, Abel would go to heaven. But, Cain would be stuck on Earth with a mark (later known as the Mark of Cain). He could never age, and he could never love. He soon became a monster, killing for the joy of the kill.

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.

A hickey or love-bite. Named after the biblical mark placed on Cain after he killed his brother... 'cause when you've got it, everyone knows what went down last night.

The Mark of Cain appeared on Cain when he took his brother Abel's life as punishment. In modern terms, people have a certain look about them that tells you they have bodies behind them. Blood is on their hands and it can never be hidden no matter how hard they try to hide it. It is wise to be able to know the look and keep your distance.