Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Italian doctor, astronomer and philosopher Luigi Lilio (also known as Aloysius Lilius).
The calendar was introduced in 1582 and has become the most widely used civil calendar in the world.
The Gregorian calendar's predecessor, the Julian Calendar, was replaced because it was too inaccurate. It did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year.
The 434th anniversary of the introduction of the Gregorian calendar is being celebrated with a Google Doodle on October 4, 2016. Here’s what you need to know about the calendar shared by Heavy.com.