The Solemnity of Mary, Mother of God is a liturgical celebration observed on January 1st. It is a holy day of obligation for Catholics, meaning that Mass attendance is required (though the Mass obligation is sometimes waived by the bishop for various reasons; when in doubt, check with your parish.)
The use of the word “Solemnity” here is not a statement about Mary’s personality. It is a designation used for certain days within the liturgical (church-based) calendar of the Church. Solemnities are the highest rank of liturgical celebration, higher than feast days or memorials. By celebrating a solemnity dedicated to Mary’s motherhood, the Church highlights the significance of her part in the life of Jesus, and emphasizes that he is both human and divine.
Though New Year’s Day may seem more like a day for football and hangovers than for Mary, there’s a beautiful spiritual significance in celebrating her during the heart of the Christmas season. Pope Paul VI, in his apostolic exhortation Marialis Cultus (1974), called the Solemnity of Mary “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.”