A list of events that occurred on May 21 related to Six Flags.
- 1907 – John C. Allen is born.
- 1965 – Sally Boyden is born.
- 1988 – Ninja opens at Six Flags Magic Mountain.
- 1994 – The Right Stuff: Mach 1 Adventure opens at Six Flags Great Adventure.
- 2005 – Kingda Ka opens at Six Flags Great Adventure.
- 2008 – The Dark Knight Coaster opens at Six Flags Great America.
- 2011 – Kidzopolis opens at Great Escape.
- 2015 – Twisted Colossus Media Day at Six Flags Magic Mountain.
- 2021 – Six Flags Darien Lake opens for the 2021 season.