Christmas Day celebrates the birth of Jesus which is presumed to take place 25 December 1 BC, but no one really knows if that's accurate. It is a holiday that takes place in many different countries around the world, but not always on the 25th.
Most people celebrate with mistletoe and caroling around the house or neighborhood. Present giving is another common celebration and almost all people have Christmas Day off.
The holiday started out as a Christian tradition to celebrate the birth of Christ along with the Winter Solstice way back in the days of the Holy Roman Empire. But as religion became less important, secular ideas of Santa and gift giving became part of the norm for this day.