Fatto generale: 21909 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21909 is a boring number.
Trivia: 21909 is an uninteresting number.
Data storica: December 25th is the day in 1974 that Marshall Fields drives a vehicle through the gates of the White House, resulting in a four-hour standoff.