December 21 is the 355th day (356th in leap years) of the year. There are 10 days remaining.
- 1861 — Ane Cathrine Andersdatter died.
- 1952 — Larry Eyler is born.
- 1966 — Oleg Rylkov is born.
- 1978 — John Wayne Gacy is apprehended.
- 1992 — Antanas Varnelis is apprehended.