Christmas Eve is the evening or entire day preceding Christmas Day, the festival commemorating the birth of Jesus of Nazareth. Christmas Day is observed around the world, and Christmas Eve is widely observed as a full or partial holiday in anticipation of Christmas Day. 

Christmas or Christmas Day (Old English: Crīstesmæsse, meaning "Christ's Mass") is an annual festival commemorating the birth of Jesus Christ, observed most commonly on December 25 as a religious and cultural celebration among billions of people around the world

Together, both days are considered one of the most culturally significant celebrations in Christendom and Western society.