general_fact: 8420 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
math_fact: 8420 is the number of symmetric ways to fold a strip of 20 stamps.
trivia: 8420 is an uninteresting number.
historical_date: January 19th is the day in 1883 that the first electric lighting system employing overhead wires, built by Thomas Edison, begins service at Roselle, New Jersey.