Today, February 2nd, is the day of the Feast of the Purification, or the Feast of the Presentation, commonly called Candelmas.
Today is a day of purification, renewal, and hope. On this day, exactly 40 days after Christmas, we commemorate Mary's obedience to the Mosaic law by submitting herself to the Temple for the ritual purification, as commanded in Leviticus.
The hymn heard here is the proper Office Hymn for the Day. An office hymn has formed a part of the Daily Offices of western Christians since the time of St. Ambrose in the fourth century. Ambrose is credited with beginning the practice of singing hymns in his cathedral, and the earliest surviving Latin office hymns are attributed to him. The office hymns at noonday and compline are chosen for the time of day, as are those for Morning and Evening Prayer on ordinary weekdays. Proper hymns are used during the major seasons of the liturgical year and on festivals. In the Prayer Book offices of Morning and Evening Prayer, the office hymn is typically sung after the opening Preces and Responses, immediately before the singing of the psalm/s of the day.
There is another "incorrect" tradition of singing the Office Hymn immediately before Magnificat at Evensong, but this breaks the flow of the service.
The melody of this hymn comes from the "Bayeux Antiphoner" of 1739, and it is harmonized by Ralph Vaughan Williams (1872-1958). The name of the tune is "Diva Servatrix."
The text is from the Editors of the "New English Hymnal," and is based upon the Latin text, "Quod chorus vatum", circa 9th century.
I've played in this in a cathedral manner, but deliberately have "scaled down" the volume and feel of it. I've stuck very closely to Vaughan Williams' harmonies, and there is one full verse for the introduction.