Each liturgical season has a special hymn to Our Lady called the Marian antiphon. It is usually the final prayer of the day said after Compline or Night Prayer. The antiphon for Advent is called Alma Redemptoris Mater (Loving Mother of our Redeemer)
This is a choral version I prefer the plain chant but this is beautiful also
I put the full text of the antiphon under the clip...
Alma Redemptoris Mater
quae previa caeli
porta manes, et stella maris, succerre cadenti
surgere qui curat, populo
Tu quae genuisti natura mirante, tuun sanctum genitorem
Virgo prius ac posterius
Gabrielis ab ore
sumens illus Ave,
Loving mother of our Redeemer,
gate of heaven, star of the sea,
Assist your people who have fallen yet seek to rise again
To the wonderment of nature you bore your creator
yet remained a virgin after as before.
You who received Gabriel's joyful greeting
have pity on us sinners.