The Catholic Encyclopedia, 1913 Volume III, 1908, Christmas:
“The well-known solar feast, however, of Natalis Invicti, celebrated on 25 December, has a strong claim on the responsibility for our (Catholic) December date. For the history of the solar cult, its position in the Roman Empire, and syncretism with Mithraism, (see Cumont's epoch-making "Textes et Monuments" etc., I, ii, 4, 6, p. 355. Mommsen ‘Corpus Inscriptionum Latinarum, 12, p. 338’) has collected the evidence for the feast, which reached its climax of popularity under Aurelian in 274.”
“The well-known solar feast, however, of Natalis Invicti, celebrated on 25 December, has a strong claim on the responsibility for our (Catholic) December date. For the history of the solar cult, its position in the Roman Empire, and syncretism with Mithraism, (see Cumont's epoch-making "Textes et Monuments" etc., I, ii, 4, 6, p. 355. Mommsen ‘Corpus Inscriptionum Latinarum, 12, p. 338’) has collected the evidence for the feast, which reached its climax of popularity under Aurelian in 274.”