Fatto generale: 21026 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21026 is a boring number.
Trivia: 21026 is an unremarkable number.
Data storica: July 25th is the day in 1261 that the city of Constantinople is recaptured by Nicaean forces under the command of Alexios Strategopoulos, re-establishing the Byzantine Empire.