Please read the entire post, because the 2nd half is important here.
This year, we have two obligatory Solemnities back-to-back.
Saturday is Dec. 8 The Immaculate Conception
Sunday is Dec. 9, the 2nd Sunday of Advent
(yes, everyone knows this, but since posts & threads get quoted and resurface years later, I felt the need for this little introduction)
The Solemnity of the Immaculate Conception begins at the time of Evening Prayer on Friday evening.
The 2nd Sunday of Advent begins at the time of Evening Prayer on Saturday evening–and at the same time, the feast of the Immaculate Conception ends.
Each solemnity carries one obligation to attend Mass, completely separate and distinct from each other. We have two obligations–there’s no way around this.
The obligation to attend Mass on the Immaculate Conception is fulfilled either on the day of the feast, or the evening before.
The claim (unfortunately often made on CAF) that canon law states that a day runs from midnight to midnight is simply untrue. I say again, it is untrue.
Canon law does say this
“Can. 202 §1. In law, a day is understood as a period consisting of 24 continuous hours and begins at midnight unless other provision is expressly made…”
Read that again “…unless other provision is expressly made”
Canon 1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the preceding day satisfies the obligation of participating in the Mass.
“The feast day itself” of the Immaculate Conception ends on Saturday evening. It does not continue to midnight. The reason is because the 2nd Sunday of Advent is ranked above the Immaculate Conception in the Church’s calendar—the higher ranked feast takes precedence. (See the Universal Norns on the Calendar)
In order to fulfill the obligation for the Immaculate Conception, one must attend Mass from Friday evening to Saturday afternoon (since Sunday begins at evening).
Attending Mass on Saturday evening fulfills the obligation for the 2nd Sunday of Advent. It does not fulfill the obligation for the Immaculate Conception.
This is not (note the negative) because the readings are different or the opening collect is different, or the hymns etc. etc. It’s because the day of the Immaculate Conception will have ended. This is
not the same situation we often encounter when someone asks about a wedding Mass on Saturday at 7 PM or an ordination Mass on Sunday at 11 AM. It’s different because a Mass on Saturday evening is not a Mass on the Immaculate Conception. Note that it is not “on” the Immaculate Conception; instead of not “for” the Immaculate Conception.
What I’ve cited above is the canon law and the liturgical law of the Church.
Now, to continue with part 2.
The above is how the laws of the Church regulate the days.
What follows is my opinion.
A person who attends only the Saturday evening Mass knowing that the Mass will be both
for the 2nd Sunday of Advent and
on the 2nd Sunday of Advent has not met the obligation for the Immaculate Conception.
However, a person who unknowingly or unwittingly attends the Mass on Saturday evening in an attempt to fulfill the Immaculate Conception obligation would not be at fault for missing Mass on the Holy Day. This might happen if the priest errs in the Mass schedule and celebrates the wrong Mass in the evening—the fault is that of the priest, not the attendee. Such a person would fulfill the Sunday obligation, but not the Holy Day obligation, but without fault.
If it is physically or morally impossible to attend Mass from Friday evening to Saturday afternoon, yet one is able (and does) attend Mass on Saturday evening and Sunday morning, that person would be excused from the Immaculate Conception obligation but fulfill the Sunday obligation.