Christmas Eve is basically refereed to the evening, which takes place just before the day of Christmas or it may also be referred to the entire day followed by the day of Christmas. The day is celebrated as the holiday in some major parts of Unites States. 
 
The day of Christmas Eve is considered as the integral part of Christmas Day, since it directly attributes itself only to the day of Christmas. 

This is the reason that why Christmas Eve together with the Christmas Day is celebrated as the most important pair of cultural festivals, which takes place not just in USA but across all European and Western culture.

Well, just like the Christmas Day the Christmas Eve is also an yearly occurring festival, which takes place on the specific day that is one day prior to Christmas. 
 
Christmas Eve basically starts from the sunset and before sunset people generally prepare for night and the next day Christmas celebration. It is believed that Jesus Christ was actually born on midnight due to which the concept of Christmas Eve got wide cultural practice. 
 
The actual celebration of Christmas eve actually begins from the Christmas Eve since the long cultural practice of Christianity. The concept of Christmas is based on "There was evening and There was morning" which means the night or evening was the actual moment of Jesus Christ's birth while the next day morning or day is also the part of birth. 
 
 There are the diverse ways of celebrating the Christmas Eve across the world as some people go to the churches for the prayers while the others celebrate the day as the part of Christmas Day by celebration.   

 
So, the celebration of Christmas Eve varies from one place and people to the others. Some celebrate the festival as the landmark night of Jesus Christ birth by offering prayers in the church. Others may celebrate it as the mini Christmas Day to celebrate it just like the tradition of Christmas.