Fatto generale: 1621 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1621 is a prime that remains prime when preceded and followed by one, two, three, or four 3's.
Trivia: 1621 is an unremarkable number.
Data storica: June 8th is the day in 1794 that Robespierre inaugurates the French Revolution's new state religion, the Cult of the Supreme Being, with large organized festivals all across France.