What is the Gregorian calendar?

The Gregorian calendar, used by most countries in the western world, was initially decreed by Pope Gregory XIII on 24 February 1582. The Gregorian calendar was first proposed by Aloysius Lilius because the mean year in the Julian Calendar was slightly long, causing the vernal equinox to slowly advance earlier in the calendar year.

On 5 October 1582, the Gregorian calendar was actively adopted in the western world for the first time. It needed an adjustment to correct 11 accumulated days from the Julian calendar. The day after Thursday, 4 October 1582 was Friday, 15 October 1582, effective in most Catholic countries such as Italy, Poland, Spain and Portugal. Non-Catholic countries such as Scotland, Britain and its colonies still used the Julian calendar up until 1752, and some Asian countries were still using the Julian calendar up until the early twentieth century.