Dom Germain Morin has shown that at Capua, in the sixth century, and also in Spain, Mass was celebrated during Lent only on the Wednesday and the Friday. It is probable that a similar rule, but including the Monday also, obtained in England in the days of Bede or even later (see "Revue Benedictine", 1891, VIII, 529). At Rome we also know that down to the time of Pope Gregory II (715-731), the liturgy was not celebrated on Thursdays. In the East, Canon xlix of the Council of Laodicea (365?), laid it down "that it is not lawful to offer bread in Lent except on the Saturday and the Lord's day", while the Council of Constantinople (in Trullo), in 692, speaks explicity of the liturgy of the presanctified and appoints it to be celebrated on all days of Lent, except the Saturday, the Sunday, and the feast of the Annunciation.
Prior to the first part of the eighth century, the church of Rome shared the custom of Byzantium and Milan in abstaining from the celebration of Mass on a regular basis during Lent; all of the Thursdays of Lent were “aliturgical”, (with the obvious exception of Holy Thursday,) as were the Saturday before the first Sunday of Lent, and the Saturday before Palm Sunday. (The term aliturgical refers, of course, only to the Eucharistic liturgy, not to the Divine Office.) The Würzburg Lectionary, the oldest surviving lectionary of the Roman Rite, represents the Roman tradition of the mid-seventh century, and contains the oldest list of Lenten Stations; in it, we find no stations or readings appointed for these days.
Why did this change?
The collection of papal biographies called the Liber Pontificalis tells us that Pope St. Gregory II (715-731) changed this custom, “establish(ing) that on Thursday in the Lenten season there should be a fast and the solemn celebration of Mass, which the blessed Pope Melchiades (311-314) had prohibited.” Under Melchiades himself, it is also noted that “the blessed Gregory (the Great) in arranging the offices (i.e. liturgies) left Thursday within Lent empty.” This is the reason why even in the Missal of St. Pius V, the masses of the Thursdays of Lent have no proper chant parts, borrowing their introits, graduals, offertories and communions from other masses; the respect for the tradition codified by Gregory the Great was such that it was deemed better not to add new pieces to the established repertoire. The two formerly aliturgical Saturdays, on the other hand, simply repeat the Gregorian propers from the previous day, indicating that their masses were added by a different Pope.
The question naturally arises, however, as to why the Pope felt the need to change the long-standing tradition. The answer seems to be in the controversies between the Popes of that era and the Byzantine Emperors over the Quinisext Synod.
The distinct and condensed formula of the Agnus Dei itself, however, was not apparently introduced into the Mass until the year 687, when Pope Sergius I decreed that during the fraction of the Host both clergy and people should sing the Agnus Dei: "Hic statuit ut tempore confractionis dominici corporis Agnus Dei, qui tollis peccata mundi, miserere nobis, a clero et a populo decantetur" (Liber Pontificalis, ed. Duchesne, I, 381, note 42). Duchesne, accepting the view of Sergius's reason propounded by Cardinal Bona, says: "il n'est pas defendu de voir, dans ce décret de Sergius, une protestation contre le canon 82 du concile in Trullo, qui proscrivit la representation symbolique du Sauveur sous forme d'agneau".