Fatto generale: 639 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 639 is a number n whose 5^{th} root has a decimal part that begins with the digits of n.
Trivia: 639 is a boring number.
Data storica: September 30th is the day in 1907 that McKinley National Memorial, final resting place of assassinated U.S. President William McKinley and his family, dedicated in Canton, Ohio.