This post is for my fellow Catholics of the Roman Rite in the United States.
Here is something that should be addressed on this forum, especially since questions will certainly arise closer to the time.
The Immaculate Conception and The Second Sunday of Advent, 2018:
December 8 is the Solemnity of the Immaculate Conception of the Blessed Virgin Mary. However, this year (in 2018), the Immaculate Conception remains a Holy Day of Obligation, even though December 8 falls on a Saturday.
In general, the Immaculate Conception remains a Holy Day of Obligation, even if December 8 naturally falls either on a Saturday or on a Monday. This is because Mary, under the title of the Immaculate Conception, is the Patroness of the United States.
In the time frame, from Friday, December 7, to Sunday, December 9, we are obligated to attend two separate Masses for two distinct obligations: We are obligated to attend one Mass, in order to fulfill our obligation for the Holy Day (The Immaculate Conception), as well as another Mass, in order to fulfill our Sunday obligation (The Second Sunday of Advent).
Since these two obligations are distinct, they must be fulfilled by attending two separate Masses. One cannot fulfill both obligations, at once, by only attending one Mass. It is not a “two-for-one”/“two-fer.”
Which Masses (i.e., the readings, the propers, the texts, etc.) will be for the Immaculate Conception, and which will be for the Second Sunday of Advent?
Friday, December 7 (at 4 p.m. or after) - Saturday, December 8 (before 4 p.m.): The Immaculate Conception
Saturday, December 8 (at 4 p.m. or after) - Sunday, December 9 (all day): The Second Sunday of Advent
Q.: If one first attends a Mass on the evening of Saturday, December 8 (between 4 p.m. (inclusive) and Midnight (exclusive)), then can they attend that Mass to fulfill their obligation for the Holy Day, even though the readings, the propers, and the texts of that Mass would be for the Second Sunday of Advent?
A.: Yes. However, because we are obligated to attend two separate Masses for two distinct obligations, that person must still attend another Mass, either later on, that same evening (i.e., after the first one that they attended), or some time, the following day, in order to fulfill their Sunday obligation. Again, both obligations cannot be fulfilled, at once, by only attending one Mass. The two obligations are distinct. Hence, they require attendance at two separate Masses.
To sum it up, there are six ways to fulfill both of these obligations to attend Mass:
Option #1: Mass on Friday evening (at 4 p.m. or after) + Mass on Saturday evening (at 4 p.m. or after)
Option #2: Mass on Friday evening (at 4 p.m. or after) + Mass on Sunday
Option #3: Mass on Saturday (before 4 p.m.) + Mass on Saturday evening (at 4 p.m. or after)
Option #4: Mass on Saturday (before 4 p.m.) + Mass on Sunday
Option #5: Mass on Saturday evening (between 4 p.m. (inclusive) and Midnight (exclusive)) + Another Mass on Saturday evening (Of course, after the first Mass that they attended)
Option #6: Mass on Saturday evening (between 4 p.m. (inclusive) and Midnight (exclusive)) + Mass on Sunday