general_fact: 21909 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
math_fact: 21909 is a boring number.
trivia: 21909 is an uninteresting number.
historical_date: December 25th is the day in 1991 that Mikhail Gorbachev resigns as president of the Soviet Union (the union itself is dissolved the next day).