April 21 is the 111th day of the year (112th in leap years). There are 254 days remaining.
- 1831 — Gesche Gottfried died.
- 1911 — Frederick Jahns died.
- 1947 — Robert Black is born.
- 1951 — Staf Van Eyken is born.
- 1957 — Patrick Salameh is born.
- 1972 — Mark Scott is murdered by Dean Corll.
- 1973 — Sara Taylor "Sally" Hallett is murdered by Ed Kemper.
- 1984 — William Paul Thompson is apprehended.
- 1986 — Lora Heedick is murdered by Roger Kibbe.
- 1992 — Robert Alton Harris died.
- 2010 — Francisco Acevedo is apprehended.
- 2011 — Gary Hilton is sentenced to death in Florida, United States.
- 2016 — "Nikopol Maniac" is apprehended.