While Christmas has roots far in the past, many of our traditions in the English world were introduced by the Victorians. This was the period that moulded Christmas into important celebration is it today, deciding on the themes we recognise (charity, goodwill, gift giving etc), the traditions (many drawn from Germanic ones) and even the commercialism (cards, Christmas crackers etc) This BBC link should be useful.
As for the pre-Christian background: check out the wikipedia page for Christmas. "Modern Christmas customs include: gift-giving and merrymaking from Roman Saturnalia; greenery, lights, and charity from the Roman New Year; and Yule logs and various foods from Germanic feasts". Actually, read all of that section.
This Britannica article goes into some other detail. It also mentions Mithras.
(There is lots of info online, but I hesitate to link to non-wikipedia or encyclopaedia articles on such a touchy issue). This is decent where it provides references but has a massive agenda.
Bonus: If you think Christmas is mostly about the food (who can blame you) you'll find this answers everything.
Christmas was originally a pagan tradition in northern Europe where they were celebrating that the sun had started to rise again. Winters were difficult to survive in those days with no lights, bad clothing, worse housing and heating and sometimes not even enough food etc. So winter and its end was a much bigger deal back then than it is today.
Then when Christianity came to northern Europe it was adapted to the already existing traditions to ease the conversion, which included Christmas, or Jul as it was called.