Fatto matematico: 9966 is the largest 4-digit strobogrammatic number.
Trivia: 9966 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: April 14th is the day in 1294 that Temür, grandson of Kublai, is elected Khagan of the Mongols and Emperor of the Yuan Dynasty with the reigning titles Oljeitu and Chengzong.