O Oriens,
splendor lucis aeternae, et sol justitiae:
veni, et illumina sedentes in tenebris, et umbra mortis.

O Sunrise,
splendour of light eternal and sun of righteousness:
Come and enlighten those who dwell in darkness and the shadow of death.

The people who walked in darkness have seen a great light; those who dwelt in a land of deep darkness, on them has light shined. Isaiah 9:2

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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.