As I was looking ahead at some preparations for work, I noticed that one of the few exceptions to my rule on whether you have to go to Mass will occur this year. I believe this will only be the case in the United States (or any other area where the Immaculate Conception is the patron of the region).
Sadly, as many of you know, with the liturgical reforms following Vatican II, options were allowed for local bishops' conferences to adapt some laws regarding holy days of obligation. Thus, I think all of the bishops' conferences in the English speaking world have now removed the obligation if the holy day of obligation falls on a Saturday or a Monday. Personally, I don't think this really benefits the faithful, but for whatever reason, the bishops allow it.
So, in general the rule goes: "you have to go to Mass on Sundays and holy days of obligation, but you never have to go to Mass two days in a row."
In the U.S.A. the only exceptions are Christmas and the feast of the Immaculate Conception, because Mary, under that title, is the patroness of the United States. The obligation for these is NEVER removed.
This year, the feast of the Immaculate Conception falls on a Saturday, and the obligation is NOT abrogated. This will be a bit confusing nowadays, because so many people regularly fulfill their Sunday obligation by going to the Sunday vigil Mass on Saturday (a quasi-liturgical abuse in my opinion, but that is for a different post). So, basically this will be like having two Sundays in a row, and you have the awkward confusion of what happens Saturday evening: is it still the Solemnity of the Immaculate Conception, or is it now the vigil of Sunday? Will priests have Saturday evening Mass of the Immaculate Conception, or will they have the Sunday Vigil Mass; another way of putting it: which day "trumps" the other? Technically a Solemnity trumps even Sunday. BUT a Sunday of Advent is a bit more "important" than a regular Sunday and I think it would even "bump" the Immaculate Conception if it fell on a Sunday in Advent (which I think happened back in 2002). So, it is just absolutely confusing. Even on a different topic, do you say Evening Prayer II of the Immaculate Conception, or do you pray Evening Prayer I of the 2nd Sunday of Advent? Usually the only way to find out is to get a hold of an ordo and see what it says. Unfortunately, I have not yet gotten our ordo for the new liturgical year (which begins with the beginning of Advent, December 2nd this year).
Off the top of my head I would say that the 2nd Sunday of Advent liturgically "trumps" the Solemnity of the Immaculate Conception. (Just wait until Lent and Easter of 2008 when the early date of Easter bumps all sorts of things around: Saint Joseph gets "bumped" to the Saturday before Palm Sunday and the Annunciation gets "bumped" to the Monday after Easter Week).
Pastorally, I would say the safest thing would be to keep the usual Sunday (and Sunday Vigil) Masses the same. The problem is that often "pastoral" is used as an excuse to do something incorrectly. Rather than catechize or educate the faithful, pastors will be "pastoral" and just adapt something to what the uneducated faithful would think or do.
And just to throw out a quick rant: I think this is why the faithful are so ignorant and apathetic, because everything is made so easy for us that nowadays we Catholics really only have to make a minuscule effort to practice our faith. We really only think that we have to go to Mass on Sundays and (not even all of the) holy days of obligations and go to Communion once a year (and how many of the faithful even know this?). (Yes, I know there is much more to living the faith, but I just wonder how many "practicing" Catholics' only act of religion is going to Mass weekly and on Christmas and Easter.)
Anyway, just giving you a heads up. You have been warned. LOL.