O Emmanuel, Rex et legifer noster,
exspectatio Gentium, et Salvator earum:
veni ad salvandum nos, Domine, Deus noster.

O Emmanuel, our king and our lawgiver,
the hope of the nations and their Saviour:
Come and save us, O Lord our God.

Therefore the Lord himself will give you this sign: the virgin shall be with child, and bear a son, and shall name him Immanuel. Isaiah 7:14

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During the evening prayer of the Liturgy of Hours, which is also known as Vespers, the Church prays the beautiful prayer of Mary known as the Magnificat. Preceding the Magnificat, there is an antiphon which is simply a short verse which connects the prayer to the particular feast day or liturgical season. On the last seven days of Advent, from December 17th through December 23rd, the antiphon for Vespers is one of the so-called "O Antiphons". It is related to a particular title of Christ's, and the antiphon ends with a plea for the Messiah to come. Click here to read more about the O Antiphons.