The word Christmas comes from Cristes maesse, or "Christ's Mass." It is the time when Christians celebrate the birth of Jesus.
As it does not say in the Bible, no-one knows when Jesus was born, neither which day nor even year. One reason for this is that at the time Jesus was born people used the Roman calendar which was different to ours. It is now believed that Jesus was born between 7BC and 4BC.
Around 350 years after Jesus birth, Pope Julius 1 chose 25 December as the date of the Nativity. This date seemed to be a good choice because many people had from early times celebrated other winter festivals around this time. There was the Roman Saurnalia where Saturn, the god of Harvest, was honoured by merrymaking. In Northern Europe, Yule, giant logs were trimmed with ribbons and green plants and burned in honour of the gods who it was hoped would make the sun shine more brightly.
Both of these festivals were around the time of the winter solstice which is the day in Northern parts of the world that is the shortest of the year. With our modern instruments we now know this to happen on 21 December.
Christmas Day is a public holiday in Christian countries. Festivities include the exchanging of presents. To children this is probably the most exciting day of the year when if they have been good they can open the gifts left by Santa Claus.