Who began the observance of Christmas?

Answer 1

During the first centuries of the common era, a large minority of pagans converted to Mithraism or Christianity, with some also converting to Judaism. Mithraism and Christianity inevitably vied for supremacy in the Roman Empire. December 25th was celebrated as the birth date of Mithras, but there was, as yet, no official date for the birth of Jesus. In 320, Emperor Constantine called the Christian Council of Nicaea to establish some common beliefs and formalities for the Church which had gained imperial patronage. One of the decisions of the Council was to expropriate the date 25th December as the official birhday of Jesus. Thus began the observance of Christmas.