The Gregorian leap year rule: divisible by 4, except centuries not divisible by 400
- In the Gregorian calendar, years divisible by four are leap years, except end-of-century years, which must be divisible by 400.↗ 2 sources
- The Gregorian calendar was instituted by Pope Gregory XIII in 1582 through the bull Inter gravissimas, dated 24 February 1582.↗ 2 sources
- At the 1582 introduction, Thursday 4 October 1582 was followed by Friday 15 October 1582.↗ 1 source