Its Just a word about Gregorian Calendar

The Gregorian calendar, or New Style calendar, was slowly adopted throughout Europe. It is used today throughout most of the Western world and in parts of Asia. When the Gregorian calendar was adopted in Great Britain in 1752, another correction of an 11-day discrepancy was made; the day after September 2, 1752, became September 14.

The British also adopted January 1 as the day when a new year begins. The Soviet Union adopted the Gregorian calendar in 1918, and Greece adopted it in 1923 for civil purposes, but many countries affiliated with the Greek church retain the Julian, or Old Style, calendar for the celebration of church feasts.

The Gregorian calendar is also called the Christian calendar because it uses the birth of Jesus Christ thought in 1 BC, as a starting date. Dates of the Christian era (see Chronology) are often designated AD (Latin: anno domini, "in the year of our Lord") and BC (before Christ).
As indicated, the Gregorian calendar is basically a Christian calendar. The official Christian church calendar is a table containing the holy days, saints' days, and festivals of the church, with the dates of the civil calendar on which they occur. These include the fixed feasts, such as Christmas, and the movable feasts, which depend on the date of Easter. The most important early church calendar was compiled by Furius Dionisius Philocalus about 354. After the Reformation, the German Lutheran church retained the Roman calendar, as did the Church of England and some other Anglican churches. The calendar of the Protestant Episcopal church retains only those festivals that have a scriptural origin. The principal seasons of the church calendar observed by most Christians are, in order, Advent, Christmas, Epiphany, Lent, Easter, Ascension, Pentecost, and Trinity.