General fact: 15 is the number of days in each of the 24 cycles of the Chinese calendar.
Mathematical fact: 15 is a repdigit in binary (1111).
Trivia: 15 is the age for obtaining a driver's (or learner's) permit in certain jurisdictions, including some where the age for a driver's license is sixteen.
Historical date: January 15th is the day in 1936 that the first building to be completely covered in glass, built for the Owens-Illinois Glass Company, is completed in Toledo, Ohio.