Someone mentioned to me that this year, the Feast of the Immaculate Conception (December 8 ) falls on a Sunday.

I have not verified the following, but I am also told that the SSPX calendar indicates Sunday, December 8 as the Feast of the Immaculate Conception.

But I thought that Sunday of Advent outranked any other competing feasts, in which case, the Feast of the Immaculate Conception ought to be transferred to Monday?

Is this just a mistake on the SSPX calendar, or do they actually intend to omit the 2nd Sunday of Advent?

Or am I mistaken in my understanding of the liturgical calendar?

My suspicion was that perhaps the SSPX intended to follow suit with the US bishops, but even in the conciliar church I see conflicting information, with this article explicitly stating that the Novus Ordo would be obliged to maintain the traditional Ordo in transferring the Feast of the Immaculate Conception to Monday:

https://www.patheos.com/blogs/deaconsbench/2019/10/when-december-8-falls-on-a-sunday/

Can anyone help clear this up?

Does the SSPX really intend to celebrate the Feast of the Immaculate Conception instead of the 2nd Sunday of Advent (and if so, upon what justification)?

I'm pretty sure that all first class feast days are celebrated on Sunday if that feast falls on a Sunday.

Sunday is the highest first class feast day, and outranks all others.

Here is a website for a church (of unknown affiliation) whose liturgical calendar for the traditional Latin Mass shows the Immaculate Conception properly transferred to Monday:

https://holycomforterparish.org/traditional-latin-mass/

I just checked the “Tabella Occurentiae” in the breviary, and it clearly and definitively settles the matter:

This year, the Feast of the Immaculate Conception is to be transferred to Monday.

So my original questions remain:

1) If the SSPX liturgical calendar is truly showing Dec. 8 as being the Feast of the Immaculate Conception (rather than the Second Sunday of Advent), is it just a typo, or does the SSPX actually intend to omit the Mass of Advent?

2) If the latter, on what basis does it do so?

3) Has the SSPX made this change on its own “authority,” and if so, for what reason?

...and now a traditional priest confirms that the Feast of the Immaculate Conception is to be transferred to Monday.

So...

I think that there are 2 issues involved.  1) Dec 8, as a holy day of obligation, remains on that sunday, since that is the actual day.  2) the transference of the CELEBRATION of the feast day, could be moved to Monday if the sunday of Advent outranks it.  (I don't know if it does, i'm just assuming so).
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The issue with the feast of Dec 8 being a holy day is that it's an "American" holy day.  It is not a holy day in other countries.  It is not Holy Day for the entire Latin Church, as are Christmas and Easter.  Thus, the question of if it supercedes a Sunday is decided by the bishops of the particular country.
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Sunday is the highest first class feast day, and outranks all others.

Not necessarily.  For example, the feast of Christ the King, on the last sunday of October, always supercedes the "Sunday after Pentacost". 
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I just checked the “Tabella Occurentiae” in the breviary, and it clearly and definitively settles the matter:

Is this pre 1962 or post?  Our priest mentioned that the Immaculate Conception would take place on Sunday.  I can't imagine that since America was solemnly consecrated to the Immaculate Conception in the 1800s, that the Bishops wouldn't have made it a feast of the highest 1st class.
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Now, the innovators starting post 1962 could've changed this feast day's rank, in which case, this change is part of the V2 machine.

The matter of whether or not the Feast of the Immaculate Conception is to be transferred to Monday is definitively resolved in the affirmative (according to both the traditional and conciliar liturgical norms).

The only questions remaining are:

1) Is the explanation a simple misprint on the SSPX calendar?

2) If not, by what authority or liturgical law is the transfer to Monday of the Feast of the Immaculate Conception resisted (noting that it seems the SSPX is practically the only group refusing the transfer);

3) My previous post offers one possible/plausible (but not certain) explanation.

Sean, your source is wrong.  Either it is forgetting the exception allowed by John XXIII's 1962 missal or...it is using +Benedict's hybrid/updated 1962 calendar for his "motu" latin mass.  The former is the traditional calendar; the latter is not.
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According to Wikipedia and per the 1960 calendar changes made by John XXIII for the 1962 missal, all sundays in Advent/Lent are of the highest class.  The only feast which can supercede these is the Feast of the Immaculate Conception.
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Pope John XXIII's Code of Rubrics divided Sundays into two classes. Sundays of the I class were the four of Advent, the four of Lent, the two of Passiontide, Easter Sunday, the Octave of Easter (in some traditions, called "Low Sunday"), and Pentecost.^[9] No feast whatsoever could replace the celebration of these Sundays, with the sole exception of the feast of the Immaculate Conception.^[10] All other Sundays were of II class,^[11] and outranked feasts of II class, with the exception that feasts of the Lord, whether I or II class, replaced celebration of a II class Sunday on which they happened to fall.^[12]
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Footnote 10 - from the "Rubricae Generales, 15".

The Bishops Conference declares the feast of the Patroness of the U.S. to be Sunday. So it’s Sunday not Monday, simple as that.

Wrong.  The Bishops Conf says that the feast is celebrated on Monday (see pg 7).  They use the novus ordo calendar:
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Since December 8, 2019, is the Second Sunday of Advent, the Solemnity of the Immaculate Conception of the Blessed Virgin Mary is transferred to Monday, December 9, 2019. The obligation to attend Mass, however, does not transfer. The Optional Memorial of Saint Juan Diego Cuauhtlatoatzin, December 9, is omitted this year.

December 8 fell on a Sunday thrice between my birth and the first major disruption of the Traditional Mass in November 1964: in 1946, 1957, and 1963. In 1946, I was an infant. In 1957, when I was in the eighth grade, I was an altar boy, and my recollection—not necessarily a reliable one, may I stress—is that the Mass was that of the feast with a commemoration of the Second Sunday of Advent. In 1963, I was a college sophomore, and I distinctly recall that the Mass was that of the feast with a commemoration.* Of course, in light of the rubrical change of John XXIII, it is hardly likely to have been otherwise.

As I heard Mass solely in the New York archdiocese, I have no way of knowing whether its practice obtained elsewhere. Indeed, my strong suspicion is that in this matter, as in many others, the ordo and rubrics might easily have varied from state to state or diocese to diocese.

Further apropos claims of indisplaceability of Advent Sundays, one should recall that when Christmas falls on a Monday, the Vigil Mass with only a commemoration of Advent 4 has always been celebrated on the 24th. This practice is no Vatican II innovation.
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*The thing I remember most clearly of all is the immense disappointment felt by me and my fellow Catholic school students, from first grade straight through college, whenever the Immaculate Conception or All Saints' Day fell on a Saturday or Sunday. These occurrences deprived us of a holiday.