Summary of comments of this office by Dom Schuster, in his work L'année liturgique.

The Gregorian Antiphonary only assigned twenty-four Sundays to the period extending from Pentecost to Advent. However, as the duration of the cycle depends on the date of Easter, if, because of an early Septuagesima, it is necessary to omit the offices of several Sundays after the Epiphany, their collects and readings are used after the twenty-third Sunday after Pentecost. The chants, antiphonal and responsorial, are taken from this last Sunday of the autumn cycle and are repeated at each one of the supplementary Sunday Masses. The Mass assigned to the Twenty-fourth Sunday retains its privilege of immediately preceding the Advent cycle; therefore the intercalated Masses must be placed between the Twenty-third and the Twenty-fourth, which, for this reason, has in the Missal the title of the Last Sunday after Pentecost.