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.
Practically speaking, that means all traditional Latin Masses offered on Sunday, 8 December, must be that of the Immaculate Conception of the Blessed Virgin Mary, with a second collect, secret and postcommunion from the Second Sunday of Advent's propers. This includes the Gloria, preface of the Blessed Virgin Mary and white/gold vestments at all traditional Latin Masses on Sunday. (Blue vestments may be used in traditional Latin Masses only in those places that really had an indult for their use on 8 December as of 1962.)
It also means sanctuary flowers and the playing of the pipe organ are allowed this Sunday, which are also permitted again the following Sunday, 15 December, for the Third Sunday of Advent, known as Gaudete Sunday.
The FSSP Ordo, as well as other traditional Latin Mass copies of the Ordo, state this, although without citation. If one is needed, refer to chapter 11, #91, from the general rubrics. Under "Table of Liturgical Days Arranged According to Order of Precedence," the first six categories of first class feasts are listed thusly:
1. Feasts of Christmas, Easter Sunday and Whit Sunday or Pentecost (I class with octave).
2. Triduum sacrum.
3. Feasts of Epiphany, Ascension, Blessed Trinity, Corpus Christi, Sacred Heart, and Christ the King.
4. Feasts of Immaculate Conception and Assumption of our Lady.
5. Vigil and Octave-day of Christmas.
6. Sundays of Advent, Lent and Passiontide, and Low Sunday.
For those who will miss the Second Sunday of Advent propers that begin with the beautiful Introit, "Pópulus Sion, ecce Dóminus véniet ad salvándas gentes: et audítam fáciet Dóminus glóriam vocis suæ in lætítia cordis vestri" -- fear not, as 9 December, 10 December and 14 December are third class ferial days, which means the Mass of the Second Sunday of Advent is to be offered then, minus the Alleluia verse and Credo and using the common/weekday preface.