May is the Month of Mary and so a good time to look at the Marian Antiphons, beloved not least because of the haunting and reflective plainsong melodies used with them.
Traditionally there are four such antiphons:

Alma Redemporis Mater (Advent Sunday until Candlemas)
Ave Regina caelorum (3rd February until the Wednesday in Holy Week)
Regina caeli (during Eastertide)
Salve Regina (the Monday after Pentecost until the Saturday before Advent )

Ave Regina caelorum continues to be recommended for use after Second Vespers on the Assumption and on its octave day, Our Lady, Queen but otherwise the antiphons may be used nowadays at any time of year, except for the Regina caeli, with its Easter alleluias and text.
The new Benedictine Antiphonale Monasticum, published in the last few years, allocates the texts in a traditional fashion.
Alma Redemptoris is prescribed from Advent Sunday until the Feast of the Baptism of the Lord (nowadays the end of Christmastide), and Sub tuum praesidium is for use after First Vespers on Solemnities, outside Eastertide. The Marian antiphons have been sung after Compline, at least since the 13th century. The Cistercians chanted the Salve Regina daily from 1218, the Dominicans at Bologna began to chant daily after a miracle in 1230, and the custom spread to the entire Order by 1250.
The Franciscans meeting at Metzin in 1249 also prescribed these antiphons for Compline, but did not allocate them in the The Liturgical Year -- Marian Antiphons same way as did the Roman Breviary of 1568. There continues to be a variety of practice. The Roman Catholic Archdiocese of Birmingham, for example, has prescribed use of the Marian antiphons after Lauds and Vespers, in addition to the traditional hour of Compline.

Other antiphon texts have also been added to the store. The Latin Breviary gives us Sub tuum praesidium confugimus,which, though discovered comparatively recently, is undoubtedly the oldest, since it is found on a 3rd century Egyptian papyrus.

The English language version of the Breviary also gives us O sanctissima, an anonymous text sung to a rumbustious and allegedly Sicilian tune, notably after Sunday Mass at St Stephen's, Lewisham.
(Musicians have identified this tune as a possible prototype of 'We shall overcome').

Traditionally Prayer to the Virgin Mary was strictly extra-liturgical and our forebears would have been amazed at the way we often drop a 'Hail Mary' into the Intercessions at Mass (a practice still forbidden in Ireland). What a contrast with the way the Tractarians used to recite the Angelus silently whilst the bell was rung! The feeling, at the time of the liturgical reforms in the 1960s, was that people would usually be at Mass but seldom at traditional extra-liturgical devotions such as Benediction and Rosary, still less at the Office of Compline, and that relaxing the rule a little - permitting a Hail Mary in the Intercessions - was a way of maintaining a proper devotion to the Virgin. Arguably the use of the Marian Antiphons, in personal and public prayer, also fulfils the requirement.

Source: Ebbsfleet Extra May 2008