Dominica I classis, in occurrentia, festis quibuslibet præfertur. Festum tamen Immaculatæ Conceptionis B. Mariæ Virg. præfertur occurrenti dominicæ Adventus.
Codex Rubricarum (1960), n. 15
We usually do not concern ourselves with the whimsical novelties of the novus ordo, but this Sunday may present confusion in mixed-use parishes that can be easily clarified ahead of time.
This Sunday, 8 December, is the feast of the Immaculate Conception of the Blessed Virgin Mary. The novus ordo has transfered its feast to Monday while declaring there is no obligation to hear Mass on the transfered holy day. It is not our job to explain novus ordo decisions.
The traditional Latin Mass, however, follows the calendar and rubrics in place for 1962, as specified in #28 of Universae Ecclesiae. To that end, when the first class feast of the Immaculate Conception falls on the same day as the first class Second Sunday of Advent, the feast of the Immaculate Conception is observed, and the Second Sunday of Advent is commemorated.