Devotion to Mary is not spiritual etiquette; it is a requirement of the Christian life. Looking to the Mother, we are asked to leave behind all sorts of useless baggage and to rediscover what really matters. The gift of the Mother, the gift of every mother and every woman, is most precious for the Church, for she too is mother and woman. While a man often abstracts, affirms and imposes ideas, a woman, a mother, knows how to “keep”, to put things together in her heart, to give life. If our faith is not to be reduced merely to an idea or a doctrine, all of us need a mother’s heart, one which knows how to keep the tender love of God and to feel the heartbeat of all around us. May the Mother, God’s finest human creation, guard and keep this year, and bring the peace of her Son to our hearts and to our world. And as children, with simplicity, I invite you to greet her as the Christians did at Ephesus in the presence of their bishops: “Holy Mother of God!”. Let us together repeat three times, looking at her [turning to the Statue of Our Lady beside the altar]: “Holy Mother of God!”.The video of the Holy Father's Mass can be found on the Vatican YouTube channel here. The moment at which Pope Francis invites the congregation to join him in the acclamation "Holy Mother of God" occurs just after 37:00. You might also like to look at the ending of the Liturgy, with the singing of the Alma Redemptoris Mater, and Pope Francis venerating the statue of Our Lady and, as he leaves the altar, venerating the image of the Infant Jesus.
It is worth reading the complete text of the homily, as mainstream news media appear to be following the Associated Press coverage which represents only a small section of the whole.