Roman Emperor Constantine I (Constantine the Great) is believed to have started promoting that the Roman sun festival of "Sol Invictus" coincides with Christmas. There is also some evidence suggesting Christians already marked Dec 25 for Jesus' birth for theological reasons (possibly linked to March 25th (conception/death)
Constantine's had started favoring Christianity after his vision before the Battle of the Milvian Bridge (312 AD) and issued the Edict of Milan (313 AD) for religious tolerance. His conversion and Christian patronage were gradual, aiming for empire unity, although many pagan symbols, like the sun halo, were adopted into Christian imagery symbolizing Jesus as the "Light of the World". He also declared in 321 AD, Sunday (Dies Solis - Sun Day) a day of rest. He was baptized on his deathbed, shortly before he died on May 22, 337 CE, by Bishop Eusebius of Nicomedia, a practice reflecting a common belief that baptism offered full spiritual cleansing only once.
