O Clavis David, et sceptrum domus Israel;
qui aperis, et nemo claudit;
claudis, et nemo aperit:
veni, et educ vinctum de domo carceris,
sedentem in tenebris, et umbra mortis.

O Key of David and sceptre of the House of Israel;
you open and no one can shut;
you shut and no one can open:
Come and lead the prisoners from the prison house,
those who dwell in darkness and the shadow of death.

And I will place on his shoulder the key of the house of David; he shall open, and none shall shut; and he shall shut, and none shall open. Isaiah 22:22

Of the increase of his government and of peace there will be no end, upon the throne of David, and over his kingdom, to establish it, and to uphold it with justice and with righteousness from this time forth and for evermore. Isaiah 9:7

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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.