The website HowStuffWorks has a section that explains different Christmas traditions in detail, http://christmas.howstuffworks.com/christmas.htm. They do it in a fun way, assuming how you would explain Christmas traditions to an alien who came down to earth and asked you such questions as "Why is mistletoe hanging over the front door?" to "Why do Christmas carolers walk around the neighborhood singing?" The How Stuff Works website has a "Christmas Channel" that explains how to make everything from your Christmas cards to Gingerbread cookies.
Another website, Merry-Christmas.com, http://www.merry-christmas.com/traditions.htm, includes stories behind such Christmas traditions as the song The 12 Days of Christmas, the origins of the Candy Cane, and the history of Santa Claus. One of the essays on this website has to do with Christmas Lights. Prior to the advent of the electric light bulb by Thomas Edison, people put lighted candles on their trees. This was obviously not a good idea considering Christmas trees are highly flammable. But after Edison created the light bulb, people starting putting those on their trees and then it was just a matter of time before Christmas lights were born.
Two British researchers have put together the site Christmas Archives, http://www.christmasarchives.com/index.html, which bills itself as a place for "Christmas Facts, Legends, and Customs." Some of the things you can learn about Christmas include, Christmas in Shakespeare's time, Samurai Santa, and a Polish Christmas. Most interesting to genealogists will be the sections on Christmas Accounts from Historical Documents and the Chronology of Santa Claus. From the chronology of Santa Claus you can see what your ancestor's interactions with the big guy were. For example, it states that in 1870, Santa Claus starting making the rounds to American and Canadian department stores.
A Victorian Christmas, http://www.victoriana.com/christmas/, part of the website Victoriana.com, can give you ideas about how the Victorians celebrated Christmas, gifts they gave, and how they decorated. Information is taken from 19th century magazines and other historical sources. While you are at this website, check out the entirety of the Victoriana website for information on all types of 19th century topics including decorating, travel, fashion and history. They even have a free e-magazine you can subscribe to.
The Herbert Hoover Presidential Library and Museum has an online exhibit entitled, An American Christmas: Decade by Decade at http://hoover.archives.gov/exhibits/AmChristmas/. Starting with the decade 1840, you can learn about Christmas traditions in the United States. The decade 1860 includes information about Christmas during the Civil War. Each decade description includes a tree that symbolizes that decade. For the 1960's, the tree is flocked and includes a monochromatic color scheme.
The University of Illinois Extension, http://www.urbanext.uiuc.edu/trees/traditions.html, has information on Christmas trees, everything from the history of the Christmas tree to the different kinds of Christmas trees. Other Christmassy information includes cookie recipes, a calendar of tree lighting events, and Christmas tree facts. Did you know that since 1850, Christmas trees have been sold commercially?
As you conduct your family history research don't forget to record Christmas traditions that you have been a part of during your life. These memories will be important to the next generations who will wonder what Christmas was like when you were alive. What your Christmas dinner consisted of, funny presents you gave to family members and who you celebrated with are all important. One of the memories I tell me kids about is how when I was a little girl, I thought the song, Oh, Tannenbaum was really O, Cannonball and I couldn't understand why we sang about cannonballs at Christmas!
However you celebrate Christmas, I hope it is a very Merry Christmas!