Fatto generale: 20015 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 20015 is an uninteresting number.
Trivia: 20015 is an uninteresting number.
Data storica: October 18th is the day in 320 that Pappus of Alexandria, Greek philosopher, observes an eclipse of the sun and writes a commentary on The Great Astronomer (Almagest).