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.
Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752, was followed by Thursday, 14 September 1752. Claims that rioters demanded "Give us our eleven days" grew out of a misinterpretation of a painting by William Hogarth. After 1753, the British tax year in Britain continued to operate on the Julian calendar and began on 5 April, which was the "Old Style" new tax year of 25 March. A 12th skipped Julian leap day in 1800 changed its start to 6 April. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the United Kingdom still begins on 6 April.