Fait mathématique: 6210 is the number of 5×5 matrices with non-negative entries with every row and column adding to 2.
Culture générale: 6210 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: December 31st is the day in 1907 that the first New Year's Eve celebration is held in Times Square (then known as Longacre Square) in New York, New York.