There's some excellent background on this in Fr. Francis X. Weiser, S.J., The Handbook of Christian Feasts and Customs.
I knew Fr. Weiser when he was teaching at the Weston Seminary in Massacusetts back when I was in college. I did a google search and found his book posted on-line:
http://www.neiu.edu/~history/wbook.htm
Fr. Weiser grew up in a little village in Bavaria.
PS: the early history of Advent and Christmas is laid out in Weiser, chs. 5-6.