National oldest sibling day is a day to be thankful for the oldest sibling for basically being a 3rd parent, they sacrificed many things for the young ones, and most of all, get yelled at the most.

National oldest sibling day takes place on July 15th

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.