The title of Mother of God in relation to Mary was officially adopted and defined at the First Council of Ephesus in the year 431 however even before then Mary had been called the Mother of God. It follows an unavoidable series of logic:
1. Jesus is God.
2. Mary is the mother of Jesus.
3. Therefore Mary is the mother of God.
In Greek the name is Theotokos which technically means God Bearer or She Who Gave Birth to God.
In Canada the solemnity of Mary the Mother of God is a holy day of obligation. Every country has its own list of holy days of obligation which by definition are days on which Catholics are obliged to attend Mass but which are not Sundays. In Canada there only two such days. One is Christmas day and the other is January 1st, the solemnity of Mary the Mother of God. This particular Christmas season both of these days fall on a Sunday which means you actually do not have to attend any additional masses this year.
So once again happy and joyful New Year and may 2017 be your best year yet!