Following 20 years of research and consultation, Pope Gregory XIII signed a papal bull in February 1582, ordering that the new calendar would be brought into effect on October 5 of that year.

But the Gregorian calendar established by Pope Gregory XIII in 1582 defines one year by the time it takes for Earth to travel around the sun. The two systems don't line up, so Hanukkah drifts ...

Pope Gregory XIII ordered Christian nations to adopt the Gregorian calendar in 1582, officially shifting New Year’s Day from late March to its current calendar placeholder of Jan. 1.

The Gregorian calendar, instituted by Pope Gregory XIII, went into effect in 1582. In the 1,417 years from 1583 through 3000, it’s most common for Easter to fall on April 16, with 61 instances.

A majority of Russian Orthodox churches worldwide use the Julian calendar, which celebrates Christmas on Jan. 7, instead of using the Gregorian calendar proposed by Pope Gregory XIII in 1582.

While the Gregorian calendar is the calendar model used today and was first established in 1582 by Pope Gregory XIII, Schaktman says the Hebrew calendar works differently.