Saint January 1 : Solemnity of Mary Mother of God - a Title of Mary Ratified during the Council of Ephesus in 431

Saint January 1 : Solemnity of Mary, Mother of God - 

Formally a Holy Day of Obligation but due to the pandemic, it is not possible to attend Mass in some cases.   

The title “Mother of God” is a western derivation from the Greek Theotokos, which means “God-bearer”.

The Solemnity of Mary Mother of God is celebrated on January 1st and is a Holy Day of Obligation; meaning Catholics must attend Mass except during the time of the pandemic or other illness. The Solemnity falls exactly one week after Christmas, the end of the octave of Christmas. The object of this feast is to commemorate the dignity of the Mary as Mother of God. Mary is truly the Mother of Christ, who in one person unites the human and divine nature. This title was solemnly ratified by the Council of Ephesus, 22 June, 431.

This feast was established by Pope Paul the VI in his 1974 document Marialis Cultis. 

In that document he writes:

In the revised ordering of the Christmas period it seems to us that the attention of all should be directed towards the restored Solemnity of Mary the holy Mother of God. This celebration, placed on January 1 in conformity with the ancient indication of the liturgy of the City of Rome, is meant to commemorate the part played by Mary in this mystery of salvation. It is meant also to exalt the singular dignity which this mystery brings to the "holy Mother...through whom we were found worthy to receive the Author of life."(17) It is likewise a fitting occasion for renewing adoration of the newborn Prince of Peace, for listening once more to the glad tidings of the angels (cf. Lk. 2:14), and for imploring from God, through the Queen of Peace, the supreme gift of peace. It is for this reason that, in the happy concurrence of the Octave of Christmas and the first day of the year, we have instituted the World Day of Peace, an occasion that is gaining increasing support and already bringing forth fruits of peace in the hearts of many. (MC 5, 1974)

In the Bible, this is noted, when Mary visits her cousin Elizabeth she says:

Luke 1:39-45, Mary Visits Elizabeth 42 and exclaimed with a loud cry, "Blessed are you among women, and blessed is the fruit of your womb. 43 And why has this happened to me, that the mother of my Lord comes to me?"

 The hymns used in the office of the feast also allude to Mary's dignity as the spiritual mother of men. The love of Mary for all mankind was that of a mother, for she shared all the feelings of her son whose love for men led Him to die for our redemption (Hunter, Dogm.Theo. 2, 578).At present the feast is found in the universal calendar of the church.

 The Solemnity of Mary Mother of God, falling on New Year's Day, is also designated the World Day of Peace. 

Text edited from the Catholic Encyclopedia and the Vatican Document Marialis Cultus