Portal:Religion/On this day/October 15
- 1582 - Pope Gregory XIII implements the Gregorian calendar. In Italy, Poland, Portugal, and Spain, October 4 of this year is followed directly by October 15.
- 1764 - Edward Gibbon observes a group of friars singing in the ruined Temple of Jupiter in Rome, which inspires him to begin work on The History of the Decline and Fall of the Roman Empire.