Question: Is Immaculate Conception a Holy Day of Obligation?
In the United States and other countries, the bishops have received permission from the Vatican to abrogate (temporarily waive) the requirement for Catholics to attend Mass on certain Holy Day of Obligation, when those Holy Days fall on either Saturday or Monday. Because of this, some Catholics have become confused about whether certain Holy Days are, in fact, Holy Days of Obligation. The Solemnity of the Immaculate Conception is one such holy day.
Answer: The Solemnity of the Immaculate Conception, the patronal feast of the United States, is a Holy Day of Obligation. Because this feast honoring Mary, the Mother of God, celebrates one of the most important events in salvation history, it is never abrogated, even when December 8 falls on a Saturday or a Monday.
However, if December 8 falls on a Sunday (as in, for instance, 2013), the celebration of the Solemnity of the Immaculate Conception is transferred to Monday, December 9, because Sundays in Advent take precedence over any other feast. When the celebration is transferred, rather than naturally falling on a Monday, the obligation to attend Mass does not transfer with it. See When Is Immaculate Conception 2013? for more details.