Fatto generale: 31 is the number of musical triads (12 major, 12 minor, 4 diminished, and 3 augmented).
Fatto matematico: 31 is a centered triangular number, a centered pentagonal number and centered decagonal number.
Trivia: 31 is the number of days in the months January, March, May, July, August, October and December.
Data storica: January 31st is the day in 1996 that an explosives-filled truck rams into the gates of the Central Bank of Sri Lanka in Colombo, Sri Lanka killing at least 86 and injuring 1,400.