This one caught me off guard too! I knew many of our Roman Catholic and Episcopal friends marked Mary, Mother of our Lord day in the church year, I never had.
For generations, the church, including Martin Luther, honored Mary with the title “theotokos,” from the Greek for “God-bearer,” for her role in giving birth to the Son of God. In fact, many eastern orthodox churches have large paintings of Mary and Jesus together which visualize “theotokos.” While it might be new to us, it is certainly part of a long tradition in the church!