Sunday, 24 December 2017 may be a confusing day for diocesan priests who offer the TLM, a reminder that every priest and sacristy should have an Ordo. On that Sunday, the Vigil of Christmas is the Mass to be offered all day, using the 1962 calendar, including Sunday morning. The Vigil of Christmas takes the place of the Fourth Sunday of Advent, as it has for centuries when landing on Sunday the 24th. As we mentioned last week, the novus ordo (of course) does the complete opposite.
The photo above shows the FSSP Ordo. The Vigil of Christmas entry is based on the rubrics, which state:
V – De vigiliis
28. Nomine vigiliae intellegitur dies liturgicus, qui aliquod festum praecedit, et rationem habet praeparationis ad illud. Vigilia Paschatis vero, cum non sit dies liturgicus, modo sibi proprio, seu pervigilio, celebratur.
29. Vigiliae sunt primae, secundae aut tertiae classis.
30. Vigiliae I classis sunt: a) vigilia Nativitatis Domini quae, in occurrentia, locum tenet dominicae IV Adventus, de qua, proinde, nulla fit commemoratio; b) vigilia Pentecostes.
Hae vigiliae festis quibuslibet praeferunt, et nullam admittunt commemorationem.
The key portion, translated, is "vigil of Christmas, which in occurrence takes the place of IV Sunday of Advent, hence the Sunday is not commemorated".
We won't even mention the current debate in novus ordo circles on whether a Sunday evening Christmas Eve liturgy fulfills the obligation for both Sunday, 24 December, and Christmas Day on the 25th. Thankfully such quagmires are moot in TLM communities, as the 1967 invention of an "anticipated Mass" using the following day's propers is in direct conflict with the 1962 calendar and its liturgical law, an instance where #28 of Universae Ecclesiae prevails with respect to Mass propers.
The only down-side of not hearing the Fourth Sunday of Advent propers this year is we will miss the "Rorate Caeli" Introit on that Sunday. But fear not, the hymn using similar text can be sung throughout this beautiful season of Advent.