You are correct. Dec 25th was already a pagan Roman Holiday. When Constantine blended Christianity into the pagan culture, they simply co-opted or re-titled existing holidays (holy days) to make it more palatable to the masses. The birth of Jahushua (Jesus) was most likely on the Feast of Trumpets in the Fall, Sept 29th, 2 BC. For detailed info on the birth timing use the following link: http://www.gods-kingdom-ministries.net/teachings/books/dr-luke-healing-the-breaches-book-1/chapter-10-the-birth-of-jesus/