Why the 25th?

It is not really known why Christmas day is celebrated on 25th December, but there are possibly a mixture of reasons. Both the Winter Solstice and the Saturnalia (a pagan midwinter festival from Roman times) were celebrated in December.
The shortest day of the year is between 22nd December and the 25th December. This is when the Winter Solstice is celebrated. It meant that the winter was over and that spring was on its way. The pagans worshipped the sun for triumphing over the darkness of the winter months.
The early Christians believed that as Jesus was the light, this was the right time of year to celebrate his birth. We have bought many customs from the Winter Solstice to our Christmas celebrations. Things such as Christmas Carols, holly and mistletoe were all part of pagan festivals, but now all have Christian meanings.
The first celebrations of Christmas day were in the 4th century. The festival of Saturnalia was celebrated around the 25th December, and the Roman Emperor Constantine decided to take this as Christmas day. Different religions celebrate at different times because they do not use the same calendar. The countries that follow the Orthodox Church calendar celebrate on the 7th January.

The name Christmas comes from the Mass of Christ. This mass was the only mass allowed to take place after sunset and was so performed at midnight. It was called the Christ-Mass, later shortened to Christmas..