Christmas is celebrated to remember the birth of of Jesus Christ, who Christians believe is the son of God.
The name 'Christmas' is derived from the Mass of Christ. A Mass service (communion), where Christians remember that Jesus was born to provide spiritual guidance and save human beings from sins.
The Christ-Mass service was the only, allowed to take place after sunset and before sunrise the following day. People had it at Midnight! So that's where we get the name Christ-Mass, shortened to Christmas, according to 'whychristmas'.
The day celebrated by people around the world on December 25 annually, whether Christians or not.
It's a time when family and friends meet to remember the good things they have, besides having presents.