I’m doing this off the top of my head but Sundays in Ordinary Time can be replaced by certain Feasts. You have to look at the calendar and see what Holy days fall in Ordinary Time. Those Holy Days would be celebrated instead of the Mass for the Sunday in Ordinary Time. I also believe that if The Immaculate Conception were to fall on a Sunday it would be celebrated in the Americas instead of the Sunday in Advent because Our Lady of the Immaculate Conception is the parton of the Americas. I might be wrong on that and accept any correction, since I’m not actually looking it up.
When December 8th falls on a Sunday during Advent, the feast is moved, “translated”, if you will, to Monday, December 9th. Even though December 9th is the Feast of St. Juan Diego, the Immaculate Conception takes precedence.
Nonetheless, because the Feast of the Immaculate Conception is the patronal day for the United States, I believe that the obligation remains. Although, for obvious reasons, the Vigil for the Feast cannot be celebrated Sunday evening. I think the only exception is when December 24th falls on a Sunday, like it did last year.
This is also similar to when the Feast of the Annunciation falls on a Sunday during Lent or during Holy Week. If it’s on a Lenten Sunday, the feast is moved to Monday. If it falls during Holy Week, it is moved to Easter week. Penitential Sundays (Advent and Lent) trump solemnities, I think. A few years ago, the USCCB issued a statement concerning the feast of Our Lady of Guadalupe, when December 12th fell on a Sunday. The ruling was that the feast had to be translated to Monday, December 13th. It upset quite a few people, but, the USCCB said that it had to be consistent with what the procedure is when December 8th falls on a Sunday.
Brother Rich, you are right about Sundays in
, since, for example, the Feasts of the Assumption and All Saints Day take precendence. Even the Solemnity of Saints Peter and Paul and the Birth of St. John the Baptist, also take precendence over Sundays in Ordinary Time (we had that this year on June 24th). Dioceses, parishes and religious communities can also substitute their patronal feasts when they fall on Sundays. The Paulists do that for the Conversion of St. Paul when the feast fallls on a Sunday. However, they do need a dispensation, I think.