The Seven O Antiphons for the week before the vigil of Christmas contains the pith of the Advent liturgy.  They address Christ directly under one or other of His scriptural names.  In these brief notes, I shall explain mainly their biblical background.  They were strung together from biblical phrases in the 8th or 9th century.  Liturgical gems attached to the Magnificat to honour the Mother of the Messiah, they show our longings for His coming grow more intensely as we draw nearer the feast.  The first letters of the antiphons of inverse order form the acrostic ERO CRAS, I shall be (with you) tomorrow. ... https://www.youtube.com/watch?v=oTw3Q0TmTt8

... https://www.youtube.com/watch?v=JPH-QFNmkjA