Fatto generale: 21389 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21389 is a boring number.
Trivia: 21389 is an unremarkable number.
Data storica: July 23rd is the day in 1962 that Telstar relays the first publicly transmitted, live trans-Atlantic television program, featuring Walter Cronkite.