There seems to be confusion on the feast of the Immaculate Conception this year, as December 8 falls on a Sunday. Although the novus ordo transfers the Marian feast day to December 9 this year, with conservative dioceses emphasizing the obligation to hear Mass both Sunday and Monday, it is worth noting the rubric governing the traditional Latin Mass:
"Sunday I class takes precedence over all feasts in occurrence. The feast of the Immaculate Conception of Our Lady, however, takes precedence over the Sunday in Advent on which it falls."
Chapter 3, #15 of the rubrics
Therefore, on December 8, 2024, all traditional Latin Masses should be that of the Immaculate Conception, with a commemoration (that is, a second collect, second secret and second postcommunion) for the Second Sunday of Advent.
On what is obligatory and what is not for those who attend the traditional Latin Mass, this seems to be a developing story, with some arguing the third class Feria of Advent TLM on December 9 is technically a holy day of obligation. Wild times.
