The solemnity of the Mother of God, which now coincides with the octave-day of Christmas and the beginning of the new year, was probably assigned this day because of the influence of the Byzantine Church, which celebrates the synapsis of the most holy Theotokos on December 26. This is in accordance with the Eastern practice of honoring secondary persons on the day after the feast of the principal personage (in this case, the birth of Christ). The Coptic Church celebrates this feast on January 16, but in the West, as early as the fifth century, the feast was celebrated on the Sunday before Christmas, although in France it was celebrated on January 18 and in Spain on December 18. Even before Pope Sergius introduced four Marian feasts in the seventh century (the Birth of Mary, the Annunciation, the Purification and the Assumption), the octave day of Christmas was celebrated in Rome in honor of the Maternity of Mary. Liturgical reform has restored the original Roman practice, which replaced the pagan feast of the New Year, dedicated to the god Janus, with this feast honoring the Mother of God.
source: https://www.ewtn.com/saintsHoly/saints/S/solemnityofmary.asp
