Fatto generale: 5191 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 5191 is a value of n for which s(n+1) = 2s(n).
Trivia: 5191 is a boring number.
Data storica: March 18th is the day in 1992 that White South Africans vote overwhelmingly in favour, in a national referendum, to end the racist policy of Apartheid.