Fatto generale: 31 is the number of days in the months January, March, May, July, August, October and December.
Fatto matematico: 31 is a repdigit in base 5 (111), and base 2 (11111).
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.