Gregory/Origin

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Origin
The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely accepted and used civil calendar. It has been the unofficial global standard for decades, recognized by international institutions such as the United Nations and the Universal Postal Union. The calendar was a reform in 1582 to the Julian calendar. The motivation for the reform was to bring the date for the celebration of Easter to the time of the year in which the First Council of Nicaea had agreed upon in 325. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady drift in the date of Easter undesirable. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade. The last European country to adopt the reform was Greece as late as 1923.