Christmas or Christmas Day is an annual festival commemorating the birth of Jesus Christ, observed generally on December 25 as a religious and cultural celebration among billions of people around the world. A feast central to the Christian liturgical year, it closes the Advent season and initiates the twelve days of Christmastide, which ends after the twelfth night. Christmas is a public holiday in many of the world's nations, is celebrated culturally by a large number of former or non-Christian people, and is an integral part of the Christmas and holiday season. While the birth year of Jesus is estimated among modern historians to have been between 7 and 2 BC, the exact month and day of his birth are unknown. His birth is mentioned in two of the four canonical gospels. By the early-to-mid 4th century, the Western Christian Church had placed Christmas on December 25, a date later adopted in the East, although some churches celebrate on the December 25 of the older Julian calendar, which currently corresponds to January 7 in the modern-day Gregorian calendar.