If Christmas is pagan, why do people still celebrate it? "It was a custom of the Pagans to celebrate on the same 25 December the birthday of the Sun, at which they kindled lights in token of festivity. ... Hermann Usener and others proposed that the Christians chose this day because it was the Roman feast celebrating the birthday of Sol Invictus.
It is commonly believed that the church chose this date in an effort to adopt and absorb the traditions of the pagan Saturnalia festival. First called the Feast of the Nativity, the custom spread to Egypt by 432 and to England by the end of the sixth century.