December 21 is the 355th day of the year (356th in leap years) in the Gregorian calendar. There are 10 days remaining until the end of the year. This is a frequent day for the winter solstice to occur in the northern hemisphere and summer solstice to occur in the southern hemisphere.
- 1620 – Plymouth Colony: William Bradford and the Mayflower Pilgrims land on what is now known asPlymouth Rock in Plymouth, Massachusetts.
- 1826 – American settlers in Nacogdoches, Mexican Texas, declare their independence, starting theFredonian Rebellion.
- 1907 – The Chilean Army commits a massacre of at least 2,000 striking saltpeter miners in in Iquique, Chile.
- 1937 – Snow White and the Seven Dwarfs, the world's first full-length animated feature, premieres at the Carthay Circle Theater.
- 1968 – Apollo program: Apollo 8 launched from the Kennedy Space Center, placing its crew on a lunar trajectory for the first visit to another celestial body by humans.