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The ancient Briton celebrated the goddess of the dawn around the vernal equinox, and it was close in time to the celebration of the resurrection of Jesus in Christianity.
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Historically, the specific date of Easter experienced a long period of calculation before it finally established how the annual date was calculated.
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Until the fourth century AD, the decline of the Roman Empire recognized the status of Christianity, the Roman emperor Constantine I was also baptized into the church before his death. It was this emperor who, at the first Nicaea council meeting in 325 AD, stipulated that Easter is the first Sunday after the first full moon after the vernal equinox; if the full moon is Sunday, then it is postponed until Next Sunday.
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At the first Nicaea Conference in 325, Christianity decided not to follow the Jewish calendar, but calculated the Easter date according to the vernal equinox and the full moon. The various churches have made various attempts to formulate methods for calculating the Easter date. Even a "Catholic moon" is specified so that it does not rely on astronomical observations to set "reasonable" Easter dates.
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In the Middle Ages, especially around the Renaissance, the Pope had held many meetings for the Easter issue and spent great efforts to encourage astronomy research, calendar reform, and hopes to unify the Easter dates in various regions.
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The ultimate accomplishment of the unification of the Easter Day was attributed to two people, the Italian doctor and astronomer Luigi Lilio (Aloysius Lilius, also known as Aloysius Lilius, 1510 – 1576), the German mathematician and the Jesuits. Christopher Clavius (1538-1612). This new calendar, promulgated by Pope Gregory XIII, is called the Gregorian calendar, which is the calendar that we use now.
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Therefore, in 1582, only Catholic countries such as Spain, Portugal, Poland, and Italy accepted the Gregorian calendar; Protestant countries must accept it more than one hundred years later. For example, the United Kingdom did not reform its calendar until 1752, and the Orthodox Church was even more late . At the end of 1917, Russia accepted the Gregorian calendar, and Greece did not accept the Gregorian calendar until 1923.
