Fatto generale: 21051 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21051 is an unremarkable number.
Trivia: 21051 is an uninteresting number.
Data storica: August 19th is the day in 1927 that Metropolitan Sergius proclaims the declaration of loyalty of the Russian Orthodox Church to the Soviet Union.