Medieval Roman Catholic Liturgy – In the first centuries following Christ’s death, his followers would gather to worship, pray, and remember his last supper by breaking bread and sharing wine together. These gatherings would often feature psalm singing, scripture reading, and prayer. Eventually, a formal liturgy developed, and was standardized by the ninth century.
As the liturgy was being developed and standardized, it came to revolve around a liturgical calendar, lasting from Advent (the season before Christmas) to Advent. All liturgical books, music, and scripture revolve around this calendar. Different scriptures were chosen for each mass to tell different stories from the life of Christ.
The mass is in two parts, with the liturgy of the Word (instruction and scripture lesson) being first, leading into the Eucharist, or the symbolic sharing of the Last Supper by breaking bread and sharing wine. The two parts of the mass are further divided into two categories: Ordinary and Proper. The Ordinary parts of the mass are those parts that are sung every Sunday, regardless of what time of year it is. There are only five parts to the Ordinary of the Mass: Kyrie, Gloria, Credo, Sanctus, and Agnus Dei. Every other part of the mass has a text that is dependent on the liturgical season.
The mass starts out with the processional, called the introit, which was part of the Proper of the mass, and changed according to the season. Originally, it was chanted as the priests processed to the altar, but eventually was sung after the priests arrived. Following the introit was the kyrie elesion, part of the ordinary. The Gloria follows the kyrie, except during advent, lent, and Requiem masses. The occasions which do not feature the Gloria are penitential seasons, and thus the joyful Gloria is out of place. The Collect and Epistle lesson, both proper, follow the Gloria. Following the epistle reading, the gradual is chanted. After the gradual, the alleluia is sung. Depending on the Sunday, one of the sequences may be used after the alleluia. The Gospel reading and sermon are offered, and the credo concludes the first half of the service.
The second half is the service of the Eucharist. An offertory is sung while the bread and wine are brought to the altar for consecration. The secret and preface, both proper, are both offered. Following that, the Sanctus/Benedictus is chanted, and the Lord’s Prayer is prayed. Following that, the Agnus Dei is chanted. The service concludes after postcommunion prayers and a dismissal chant are offered.
This liturgy was set in 1014, but Vatican II made some important changes to the way things are done, such as translating the service from Latin into the common language of the people. However, the basic format remains the same, and probably will remain basically the same for another nine centuries.
Source: The Development of Western Music