Fatto generale: 15 is the number of days in each of the 24 cycles of the Chinese calendar.
Fatto matematico: 15 is a triangular number, a hexagonal number, a pentatope number and the 4th Bell number.
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.
Data storica: January 15th is the day in 2005 that ESA's SMART-1 lunar orbiter discovers elements such as calcium, aluminum, silicon, iron, and other surface elements on the moon.