Because there isn’t a Saturday obligation (unless you are a priest).
Well, technically a priest is only obligated to say mass when the laity are supposed to go to mass
So, yes you are right. There is no Saturday Obligation. I should have switched it around

:blush:
What I meant to say was: Why can’t my Saturday Morning Mass fulfill my Sunday Obligation? It’s on Saturday like the Vigil.
But let’s up the ante. Let’s take the question you are raising. Does fulfilling my Christmas on Sunday (the eve before Monday) fulfill my Sunday obligation and let’s switch it around, since to have a singular answer it must work both ways.
Does it work the other way around? Does my Sunday Morning Mass fulfill my Christmas Mass since it happens the Day before?
If so, then can I go to Saturday Vigil Mass (which TECHNICALLY fulfills the Sunday Mass) to fulfill both my Sunday and Christmas Obligation? By the Logic of your question, the implications would suggest that this works.
I think I’m seeing the confusion, though, that might help clarify.
When we as Catholics speak of solemnities and the day they cover, they TECHNICALLY start the day before when we look strictly at a physical calendar marked off by squares and times. I’ll lay it out.
The Solemnity of Sunday, or one Sunday looks like this (an example):
Start Time: Saturday, December 17, 2016 at 4:00 P.M.
End Time: Sunday, December 18, 2016 at 11:59 P.M.
This Constitutes a Sunday. Same goes for a solemnity (immaculate Conception example):
Start Time: Wednesday, December 7, 2016 at 4:00 P.M.
End Time: Thursday, December 8, 2016 at 11:59 P.M.
Thus, going to Mass on Saturday Morning, in the first case, or Wednesday morning, in the second case, does not fulfill your Solemnity Obligation because it does not fall within the time frame of the day.
Make sense so far?
So now let’s take your question. 4th Sunday of Advent, 2017.
Start Time: Saturday, December 23, 2017 at 4:00 P.M.
End Time: Sunday, December 24, 2017 at 11:59 P.M.
Now Christmas 2017.
Start Time: Sunday, December 24, 2017 at 4:00 p.m.
End Time: Monday, December 25, 2017 at 11:59 P.M.
There’s the problem. The Solemnity of Christmas Overlaps with the 4th Sunday of Advent. So, in order to resolve this, we must ask the hierarchy of solemnities. We know for a fact that the highest Solemnity feasts of the Liturgical year are Christmas and Easter, with Easter being the highest one (which is why it’s called “Easter Duty”).
Because of this, Christmas trumps the 4th Sunday of Advent Solemnity. But, like we noted above, the day of the solemnity does not start until 4:00 P.M. the day before. Therefore our time adjustments should look like this.
4th Sunday of Advent 2017:
Start Time: Saturday, December 23, 2017 at 4:00 P.M.
End Time: Sunday, December 24, 2017 at 3:59 P.M.
Christmas 2017
Start Time: Sunday, December 24, 2017 at 4:00 p.m.
End Time: Monday, December 25, 2017 at 11:59 P.M.
Therefore, one cannot cover both the Sunday Obligation and the Christmas obligation in the same Mass because of the differentiation of days.
BUT to make matters more complicated, yet simpler, if you attend a Christmas Eve Mass (which still
TECHNICALLY happens on a physical Sunday)
AND a Christmas day Mass, you would have fulfilled both Obligations. Why? Because one Mass (the first one) covers the Sunday and the Second covers the Christmas.
You could also go to Mass Sunday Morning AND Christmas Eve Mass later that evening and fulfill both obligations. Why? Because the first one covers Sunday and the Second one covers Christmas (because it falls within the timeline of the Solemnity of Christmas).
WOW. I can’t believe that’s how much explaining this would take. It would have been sooooo much easier in person

Bascially, I hope you see why one Mass cannot cover two different Obligations. My preferred method (which I do when I’m home at my parish) is I go to Sunday morning Mass and then Christmas Eve.
Please, I pray that this helps you