John O’Malley (depending on the work of Gaetano Moroni) sums up the difference between the liturgy of the papal court in Rome prior to and after the Avignon papacy:
The major basilicas, especially Saint Peter’s, as well as the stational churches, all played a part in a liturgical program that over the course of any given year was distributed throughout the city. The Roman liturgy was thus public and urban as in no other city.
By the beginning of the eleventh century, however, the pope and the clerics of his household at times said Office in a private sanctuary in the papal quarters. This was the starting point of a history in which a palace liturgy, separate from the liturgies of the churches, developed. It seems clear, moreover, that the residency of the popes at Avignon in the fourteenth century modified the liturgical traditions of the papacy in ways that would have special significance for the Renaissance and even for centuries thereafter. The liturgies, deprived of their great basilicas and the other churches of Rome, became ever more confined to the chapel of the papal palace. This shift from church to palace meant a shift from a public and urban liturgy to a more private and courtly one. The significance of this change is indicated by the very title, cappelle pontificie or cappelle palatine, which in time became attached to these liturgies.
After the turmoil of the Schism, when the popes once again took up their residency in Rome, the effects of these developments continued to be felt, with the added distinction that the Vatican palace and the basilica of Saint Peter became the liturgical focus of the city to a greater degree than ever before. Nicholas V’s architectural plans and achievements for that quarter of the city meant that henceforth during the period we are considering the papal liturgies would for the most part be celebrated there. When, a few decades after Nicholas V, Sixtus IV demolished the old “cappella magna” of Nicholas III in order to construct the Sistine Chapel, we can say that a definite localization of the cappelle pontificie had been accomplished. The great feasts, such as Pentecost and All Saints, would habitually be celebrated in the basilica of Saint Peter or in one of its chapels, whereas other solemnities, such as the Sundays of Advent and Lent, would be observed in the Sistine Chapel of the palace.1