Christmas Eve is the day before 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. Together, both days are considered one of the most culturally significant celebrations in Christendom and Western society.
Christmas Day is very popular in the USA.
It says it's one of the most culturally significant celebrations in Western society, and Western society includes the USA. It's difficult because the USA is never mentioned directly in the text.