From the daily dish:
As a religious holiday, pre-modern Christmases were rather austere celebrations defined by lengthy church services. That this coincided with pagan culture’s raucous celebrations of the winter solstice was a source of great displeasure to institutional Christianity for centuries. The “kitsch” that Harris discusses (Evergreen Trees, the man from the north who brings us goodies) are pagan icons. Giving gifts and spending time with your family and friends (instead of spending the day in mass) are also holdovers from popular tradition of drinking and reveling which the church had been actively hostile to.
In short, everything WE secularists value in Christmas has been entirely appropriated by the Christian world.
I think you can also look at it as cultural evolution, we have kept the good things about the holidays i.e. Celebrating with friends and family and for the most part gotten away from spending too much time in a musty old church with people we hardly know and probably wouldn’t like if we did.
Yes, the church was unhappy with the drunken revelries around the Feast of Bacchus and so co-opted that season for the Christ Mass. And Jesus was born in springtime.
Fortunately, this was later offset by Mardi Gras before the Ash Wednesday.