The adoption of the Gregorian Calendar was an event in the modern history of most cultures and societies, marking a change from their traditional (or old style) dating system to the modern (or new style) dating system, the Gregorian calendar, that is widely used around the world today. Some states adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between; however a number continue to use a different civil calendar. For many the new style calendar is only used for civil purposes and the old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During – and for some time after – the change between systems, it has been common to use the terms Old Style and New Style when giving dates, to indicate which calendar was used to reckon them.

The Gregorian calendar was decreed in 1582 by the papal bull Inter gravissimas by Pope Gregory XIII, to correct an error in the Julian calendar. The Julian calendar had been based upon a year lasting 365.25 days, but this was slightly too long; in reality it is about 365.2422 days, and so over the centuries, the calendar was increasingly out of alignment with the earth's orbit.

Although Gregory's reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church and the Papal States. The changes he was proposing were changes to the civil calendar, over which he had no formal authority. They required adoption by the civil authorities in each country to have legal effect. The bull became the canon law of the Catholic Church in 1582, but it was not recognised by Protestant churches, Eastern Orthodox Churches, and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian churches diverged.