THE O ANTIPHONS ARE ARE AN ANCIENT AND TRADITIONAL ADVENT LITURGY

"The 'O Antiphons,' one of the oldest liturgical rituals in the church, are prayed around the world during the final days of Advent. For the seven days before Christmas, we recall in these prayers a quality of Christ that must be realized before the presence of Christ can consume the world."

The great Advent hymn, O COME, O COME, EMMANUEL is based on the O antiphons.

There is one O antiphon for each day from December 17 to 24. They are all here > http://www.benetvision.org/news_pages/oantiphons.html

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