This is one of four Marian antiphons, with following versicles and prayers, traditionally said or sung after night prayer, immediately before going to sleep. It is said from the beginning of Advent through February 1.

Alma Redemptoris Mater (O Loving Mother of our Redeemer)

O loving Mother of our Redeemer, gate of heaven, star of the sea,
Hasten to aid thy fallen people who strive to rise once more.
Thou who brought forth thy holy Creator, all creation wond'ring, 
Yet remainest ever Virgin, taking from Gabriel's lips
that joyful "Hail!": be merciful to us sinners.

Up through the day before Christmas Eve:

V. The Angel of the Lord declared unto Mary. 
R. And she conceived by the Holy Spirit.

Let us pray. Pour forth, we beseech Thee, O Lord, Thy grace into our hearts, that we, to whom the incarnation of Christ, Thy Son, was made known by the message of an angel, may by His passion and cross be brought to the glory of His resurrection, through the same Christ our Lord. Amen.