Events that occurred on 12 March.
 On Earth
- 1927 - J.R.R. Tolkien writes a letter to Ronald Ashton.
- 1958 - J.R.R. Tolkien receives Index questions from Nancy Smith.
- 2003 - "Battle of Helm's Deep", the fifth set in The Lord of the Rings Trading Card Game, is released.
- 2012 - Update 6: Shores of the Great River, the sixth free update of The Lord of the Rings Online, is released.
- 2015 - Terry Pratchett dies.
 In Arda
- T.A. 2510:
- T.A. 3019: