The feast of the Immaculate Conception of the Blessed Virgin Mary is this Sunday, 8 December 2019. Of course, the feast is observed on the actual feast day using the 1962 missal, unlike the novus ordo (which transfers the day to Monday with no obligation to attend, even in the U.S. -- patronal feast day be darned).
When this feast last occurred on a Sunday -- in 2013 -- we wrote about it here. The two important points to make are: 1) there is zero wiggle room to transfer the feast from Sunday to Monday using the 1962 missal; and 2) a commemoration must be made for the Second Sunday of Advent, even at High Masses.
On the first point, both the Immaculate Conception and the 2nd Sunday of Advent are first class feasts on the 1962 calendar. As NLM has noted, this was not always the case. The 2nd Sunday of Advent was second class until 1955 reforms promoted it to first class. The rubrics of 1960, which govern the 1962 missal, explicitly broke the first class tie between the Immaculate Conception and the Second Sunday of Advent:
Dominica I classis, in occurrentia, festis quibuslibet præfertur. Festum tamen Immaculatæ Conceptionis B. Mariæ Virg. præfertur occurrenti dominicæ Adventus.
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 of Advent on which it falls.
Given this rubric, the Mass propers for that of this Sunday are the complete Immaculate Conception propers, in white/gold (blue only where a true indult exists) vestments and with a said/sung Gloria. The preface of the BVM is used. Organ and flowers are permitted, arguably encouraged.
On the second point, a commemoration of the 2nd Sunday of Advent is to be said/sung, even at High Masses, as the commemoration is considered "privileged" (as compared to "ordinary"). This is not optional.
Therefore the first Collect said/sung this Sunday is the Immaculate Conception proper, and the second Collect said/sung is the 2nd Sunday of Advent proper. The first Secret said is the Immaculate Conception proper, and the second Secret said is the 2nd Sunday of Advent proper. The first Postcommunion said/sung is the Immaculate Conception proper. The second Postcommunion proper said/sung is the 2nd Sunday of Advent proper. Two distinct conclusions are used -- the commemoration is not tacked onto the first proper with just one conclusion. This means the servers/schola respond Amen twice (and the congregation should wait until the second Collect is done before sitting down for the Epistle at High Mass). In my experience, over half of diocesan priests who offer the TLM have no earthly idea what a commemoration is (as the novus ordo eliminated them), hence the step-by-step detail here. A teachable moment for something required at the traditional Latin Mass.
For those who offer or attend daily TLMs, the propers from the 2nd Sunday of Advent are used in their entirety on Monday the 9th, Tuesday the 10th (with a commemoration of Saint Melchiades) and Saturday the 14th. On the 11th, 12th and 13th, the Second Sunday of Advent receives a commemoration at each of the proper Masses. The Immaculate Conception, with its octave abolished, is a "one and done" liturgy on the 8th using the 1962 missal. Its propers are not used in whole or in part the rest of the week.
