D. B. Cooper Day is observed worldwide every November 24, celebrating the infamous plane hijacking that has remained an unsolved mystery for over fifty years. The day commemorates the actions of D. B. Cooper, an unidentified man who hijacked a Boeing 727 in 1971, demanded a ransom of $200,000, and parachuted out of the plane with the money. The event has inspired various theories and has become part of popular culture, influencing television shows and films. The celebration traces its roots to an annual party started in 1974 at the Ariel Store and Tavern in Washington, which evolved into a multi-day festival known as D. B. Cooper Days. After the tavern's closure, CooperCon was established by researcher Eric Ulis as an annual convention dedicated to the case, held in Seattle since 2023. Enthusiasts who cannot attend the convention celebrate D. B. Cooper Day by sharing theories and discussions about the hijacking.