Fait général: 21306 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fait mathématique: 21306 is a boring number.
Culture générale: 21306 is an unremarkable number.
Date historique: May 1st is the day in 880 that the Nea Ekklesia is inaugurated in Constantinople, setting the model for all later cross-in-square Orthodox churches.