Emperor Constantine (aka Constantine the Great) was born around 272 AD and succeeded Diocletian (244 - 311 AD) as emperor of the Western Roman Empire in 305 AD. As a polytheist, he worshipped the pantheon of Roman gods, and he partook in the celebrations of festivals honoring these pagan deities in keeping with the tradition of his forerunners. On October 28, 312 AD when he was en route to engage his arch nemesis Maxentius in the Battle of the Milvian bridge, Emperor Constantine had a vision in which a cross of light was superimposed upon the Sun with the words, “In hoc signo vinces” or “With this sign you shall conquer.” Coincidentally, he won his battle and he attributed his victory to the cross he had seen in his vision.
Thereafter, he converted to Christianity and began to champion the teachings of his newfound religion. Among other things, he, together with his co-emperor Licinius, signed the Edict of Milan (circa 313), which gave equal rights to the various religious entities in his kingdom. In 325 AD he convened the first Council of Nicaea to quell the promulgation of false doctrines in Christianity.
Even though he was a Christian convert, Emperor Constantine still had a lingering proclivity for paganism. He was instrumental in merging the story about the birth of the Christ in the gospels (termed Nativity) with prominent pagan festivals, which resulted in the creation of our present day Christmas.
Still, another was Dies Natalis Solis Invicti or “the birthday of the unconquered Sun,” which was celebrated on December 25. Historians have noted that the initial stages of Christmas was characterized by exchange of gifts; gathering of family and friends for the purpose of feasting; and celebration of birth— all of these being cardinal traits and extracts of Roman and Persian festivals dedicated to their pagan gods in antiquity. Thus, our modern day Christmas is an amalgam of prominent and notable pagan feasts and the Nativity story of the gospels.
If, for the sake of argument, Christians could commemorate the date of the Christ’s birth.