May 21 is the 141st day of the year (142nd in leap years). There are 224 days remaining.
- 1913 — John Wesley Robinson is apprehended (final time).
- 1938 — Marceline Harvey is born.
- 1939 — Roger Kibbe is born.
- 1951 — Douglas Gretzler is born.
- 1955 — Robert Edwards is born.
- 1955 — Horace Vaultz is born.
- 1960 — Jeffrey Dahmer is born.
- 1972 — Johnny Delome and Billy Baulch are murdered by Dean Corll.
- 1990 — Elissa Naomi Keller is murdered by Cleophus Prince Jr..
- 1997 — Frederick Cox is apprehended.
- 1997 — Igor Yuferev is apprehended.
- 1999 — John Bunting is apprehended.
- 2009 — Sherzod Zakirov is apprehended.
- 2010 — Suzanne Blamires is murdered by Stephen Griffiths.
- 2014 — Gregory Breeden died.
- 2015 — Gerhard Hintermeier and Erna Hintermeier are murdered by Dariusz Kotwica.