A Rubrical Note
The Mass of the twenty-fourth Sunday is always celebrated as the last Sunday of the season after Pentecost, i.e. on the Sunday before the first Sunday of Advent.
If there are actually twenty-three Sundays, the twenty-fourth Sunday is still celebrated as the last Sunday of the season. However, the Mass of the twenty-third Sunday is celebrated on Saturday, i.e. on the day before the twenty-fourth Sunday. When this takes place, the Gloria and Credo are said, the preface is that of the Holy Trinity, and Ite, Missa est is said, as on Sunday.
The Introit, Gradual, Alleluia, Offertory, and Communion chants for all of the Masses from the twenty-third Sunday after Pentecost until the last Sunday after Pentecost are those of the twenty-third Sunday.
If there are more than twenty-four Sundays, the Sunday before the last takes its collects and scriptural readings from the sixth Sunday after Epiphany.
Any additional vacant Sundays are filled working backwards from the end of the year; fifth, fourth, and third.