I know that the Immaculate Conception is transferred to December 9th this year. Is it obligatory to attend mass on that day?
This year, 2013, 8 December falls on a Sunday. For this reason, according to the Ordinary Form calendar, the celebration of the Solemnity of the Immaculate Conception has been transferred to Monday 9 December.
In these USA, the Immaculate Conception is also our national patronal feast.
For good or for ill, the bishops conference usually waives obligations of Holy Days which are too close to a Sunday (which is always a day of Obligation). We can’t have people arranging their oh so busy lives around something like Mass! Imagine such a thing! But I digress.
My understanding is that, barring particular law in some diocese, this year 9 December is NOT a Holy Day of Obligation in most dioceses.
Call your local chancery and ask, just to be sure. I have seen some variations when cruising on diocesan pages looking for their liturgical calendars. Feel free to report your findings in the combox.
In the Extraordinary Form of the Roman Rite Immaculate Conception is a 1st class feast that trumps the Sunday (which gets a commemoration). In the EF we don’t transfer Immaculate Conception to Monday (which would be a ferial day in Advent).
In other words, in the EF we know exactly what to do this year.
However, had Monday 9 December been retained as a Holy Obligation then … and this is where I briefly open and then instantly close the rabbit hole … there is a disputed question about whether evening Mass on Sunday, 8 December would satisfy both the Sunday and the Monday obligation. But this is NOT the case this year and we don’t have to worry about this, nor do we need to discuss it. I bring it up here before someone else does. Rabbit hole closed.