Vexilla Regis

The "Vexilla Regis" is a Latin hymn in long metre by the Christian poet Venantius Fortunatus, Bishop of Poitiers. It takes its title from its opening stanza:

The hymn was first sung in the procession (November 19, 569) when a relic of the True Cross, sent by the Byzantine Emperor Justin II from the East at the request of St. Radegunda, was carried in great pomp from Tours to her monastery of Saint-Croix at Poitiers. Its original processional use is commemorated in the Roman Missal on Good Friday, when the Blessed Sacrament is carried in procession from the Repository to the High Altar. Its principal use however, is in the Divine Office, the Roman Breviary assigning it to Vespers from the Saturday before Passion Sunday daily to Maundy Thursday, and to Vespers of feast of the Exaltation of the Holy Cross (September 14), and in pre-Vatican II breviary also for the feast of the Finding (May 3), and of the Triumph of the Holy Cross (July 16).

Originally the hymn comprised eight stanzas. In the tenth century, stanzas 7 and 8 were gradually replaced by new ones ("O crux ave, spes unica", and the doxology, "Te summa Deus trinitas"), although they were still retained in some places.

In the seventeenth century the correctors of the Breviary under Urban VIII revised the whole hymn in the interest of classical prosody. The Commission on Plain Chant under Pius X restored the ancient form of the text. The Graduale Romanum (1908) gives only the ancient form of the hymn, while the Antiphonary (2012) gives only the revised form. The Processionale (1911) gives both forms.

...
Wikipedia