Why is the Solemnity of the Immaculate Conception treated differently?

Well it's not any Sunday but a Sunday in advent. So the Sunday takes precedence over a solemnity.

Can't understand why it doesn't move to Saturday though. Introduction to Divine Office says that's what should happen.

Of course, in the United States, and in addition to all Sundays, we're obligated to attend Mass on the Holy Days of Obligation. There are six of them, in the United States.

Whenever January 1 (Mary, the Holy Mother of God), August 15 (The Assumption of the Blessed Virgin Mary), or November 1 (All Saints) falls either on a Saturday or on a Monday, the obligation to attend Mass is lifted.

In the United States, the Immaculate Conception remains a Holy Day of Obligation, even if December 8, itself, naturally falls either on a Saturday or on a Monday, because Mary, under this title, is the Patroness of the United States.

It's only when the Immaculate Conception is transferred to the following day (Monday, December 9) that it loses its respective obligatory status, and this only occurs in the event that December 8 falls on a Sunday (because Sundays of Advent take precedence, in the liturgical calendar).

After the Passion, Christmas is the most important moment in the life of Christ. As a result, Christmas (December 25) is always a Holy Day of Obligation, regardless of the day of the week on which it falls.

In most regions of the United States, the Ascension has been transferred to the Seventh Sunday of Easter (Hence, in such regions of the United States, the Ascension carries the Sunday obligation to attend Mass). However, in those few regions of the United States, where the Ascension continues to be celebrated on the Thursday of the Sixth Week of Easter, it is always observed as a Holy Day of Obligation.

In the United States, there are only ten states that continue to celebrate the Ascension on the Thursday of the Sixth Week of Easter: Connecticut, Maine, Massachusetts, Nebraska, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont.

Note: Everywhere else, in the United States (i.e., outside of the ten states, listed above), the Ascension has been transferred to the Seventh Sunday of Easter.

Sources:

http://www.usccb.org/beliefs-and-teachings/what-we-believe/canon-law/complementary-norms/canon-1246.cfm

https://adoremus.org/2017/11/11/q-monday-december-25-2017-holy-day-obligation-week-later-monday-january-1-2018-not-holy-day-obligation/

http://www.archindy.org/worship/holydays.html

What is so much more superlative about having the requirement to attend Mass abrogated every few years on one day for one feast?

Code of Canon Law which guides Feast Days says the following:

1246 §1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation. The following days must also be observed: the Nativity of our Lord Jesus Christ, the Epiphany, the Ascension, the Body and Blood of Christ, Holy Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, Saint Peter and Saint Paul the Apostles, and All Saints.

§2. With the prior approval of the Apostolic See, however, the conference of bishops can suppress some of the holy days of obligation or transfer them to a Sunday.

So the canon law makes it clear that the bishops' council has the authority to suppress some of the holy days of obligation or transfer them to a Sunday. And that is exactly what the United States Conference of Catholic Bishops chose to do on December 13, 1991.ref That is, they chose to remove the precept to attend Mass if a holy day of obligation (excluding Nativity) falls on a Saturday or on a Monday.

Why did they choose so? That cannot be answered by us. Only the bishops can. They might have considered the culture, local tradition etc., before deciding so.

It is to be noted that not all Conference of Catholic Bishops chose this. For example in India we do have to attend mass on two consecutive days even if a a solemnity falls on a Saturday or on a Monday.

Why is the Immaculate Conception more important than other Marian Feasts or All Saints Day (or Corpus Christi or Ascension Thursday), which are always transferred to Sundays in my diocese)?

Actually Immaculate Conception is less important than Advent Sunday and that is the reason it is transferred to the next day. But All Saints Day / Corpus Christi / Ascension are more important than Ordinary Sundays and hence the local bishops conference can decide to move them to the next Sunday of the Ordinary Time.

Is it only acceptable to celebrate a Christ/Trinity oriented feast day on a Sunday?

Yes. As the saying goes, "Our Lord's Day is the Lord of All Days". But there are exception to this rule. The ranking of the days according to their priority are:

• The Paschal Triduum and Easter Sunday
• The solemnities of Nativity of the Lord, the Epiphany, the Ascension, and Pentecost
• Sundays of Advent, Lent, and Easter season
• Ash Wednesday
• Weekdays of Holy Week up to and including Thursday
• Days within the Octave of Easter
• Other Solemnities
• Sundays of the Year

So as per the above list Sundays of Advent have more priority than the the solemnity of the Immaculate Conception, hence the solemnity of the Immaculate Conception is moved to the next day.

There is an exception to the third item on the list above. That is, if the Solemnities of a particular calendar falls on Sundays of Advent, Lent, and Easter then the local bishop can choose to celebrate them instead of the Sunday. The order of such solemnity are:

• The solemnity of the principal patron of the place, city or state
• The solemnity of the dedication or anniversary of the dedication of one's own church
• The solemnity of the title of one's own church (the mystery or saint to which it is dedicated)
• The solemnity of either the title or the founder of a religious institute

For example, if a diocese's patron saint is Mary under the Title of Immaculate Conception (like my diocese), the feast of Immaculate Conception is celebrated on December 08 in that diocese alone when the rest of the Church celebrated the Advent Sunday.

(Just of of curiosity: //festive round of Stabat Mater// Isn't Stabat Mater suppose to be a sorrowful song?? )