Vespers
The Office of Vespers in medieval Christianity was drawn up according to Benedict of Nursia’s (St. Benedict - c. 2 March 480 and c. 21 March 543 AD ) Rule of St. Benedict (Regula Sancti Benedicti) written about 530-43), Chapter xvii : "Let Vespers service however be arranged with four psalms and their antiphons, after which psalms a lesson is to be said, then a responsory, hymn, versicle, Gospel canticle, lesser litany and Lord's prayer and so to the end."
In a document of that period, vespers is described as follows: [Vespers] is composed of four psalms, a capitulum, a response, a hymn, a versicle, a canticle from the Gospel, litany, Pater with the ordinary finale, orationis, or prayer, and dismissal.
Vespers and lauds (morning prayer) are the oldest and most important of the traditional liturgy of the hours. Like the other hours, Vespers is divided into two parts; the psalmody, or singing of the psalms, forming the first part, and the capitulum and formulæ the second.
The eight canonical hours of the divine office were matins (matutinae-- during the night, after midnight), lauds (laudes-- dawn prayer), prime (prima-- the first hour, early morning prayer), terce (tertia-- third hour, mid-morning prayer), sext (sexta-- sixth hour, midday prayer), none (nona, - ninth hour, mid-afternoon prayer), vespers (vesperae -- at the lighting of the lamps), and compline (completorium-- night prayer, before retiring), which completes the cycle.
Vespers were, after mass, the most important and most solemn Office of the day. During the most important festivals, cathedrals and large chapels sometimes celebrated "solemn vespers" or "in music," hence the composition of an important repertoire, including: an anthology of psalms, hymns or magnificat intended for this use, in polyphony in the sixteenth century (Palestrina, Lassus, etc.) with soli, choirs and orchestra in the seventeenth and eighteenth centuries.
Mozart wrote for the Salzburg Cathedral a number of solemn vespers. In the nineteenth century, these compositions became exceptional, and the vespers in music, rarer, leading to greater use of the existing repertoire than new compositions.
Sources
“Vespers”, The Catholic Encyclopedia.
“Vespers, the Music of” The Catholic Encyclopedia.
Encyclopédie Larousse en Ligne
Danielle Mihram, 2020