Hecho matemático: 20816 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 20816 is a boring number.
Fecha histórica: December 27th is the day in 1966 that the Cave of Swallows, the largest known cave shaft in the world, is discovered in Aquismón, San Luis Potosí, Mexico.