Christmas is a Christian festival which celebrates the birth of Jesus Christ in Bethlehem.
December 24th is called
Christmas Eve. Eve (as in evening) means the day before. People get ready to welcome Santa Claus or Father Xmas, but they don't celebrate until Xmas Day.
December 25th is called
Xmas day. In Britain, Xmas is the most important festival of the year. People get together, exchange presents and eat a traditional meal composed of turkey and roast potatoes, followed by Xmas pudding for dessert. At 3 o'clock, the Queen makes a speech on television.
December 26th is a holiday in England because in the past it was the day when the masters gave presents (boxes) to their servants. It is called
Boxing Day.