In the normal course of the liturgical year, the solemnity of the Immaculate Conception is celebrated on Dec. 8 and it is a holy day of obligation. However, this year Dec. 8 is the Second Sunday of Advent. So, what does the Church do?
The simple answer is that the feast day gets transferred from its usual day of Dec. 8 to the next day, Monday, Dec. 9. However, what about the obligation to attend Mass on the feast when it lands on a Monday? At times, when a holy day of obligation lands on a Monday or a Saturday, the obligation sometimes is removed.
Now enter the Vatican’s Dicastery for Legislative Texts, which clarified a request from the United States Conference of Catholic Bishops noting that “the feast must be observed as a day of obligation on the day to which it is transferred.”
In short, Catholics in the United States are obligated to attend Mass for the solemnity of the Immaculate Conception on Dec. 9 this year, in addition to their regular Mass obligation for the Second Sunday of Advent.
Can someone go to a single Sunday evening Mass and fulfill their obligation for both Second Sunday of Advent and Immaculate Conception? No. Both days have important significance in the life of the Church and need to be treated as such. The feast of the Immaculate Conception is a patronal feast day in the United States. While the feast does hold importance in the Church, in the U.S. it is given even more importance and therefore the obligation to attend Mass on it is not removed.
One is obligated to attend Mass after 4 p.m. on Saturday, Dec. 7 and before 4 p.m. on Dec. 8 for the Second Sunday of Advent. However, they also would need to go to Mass again after 4 p.m. on Dec. 8 or on Dec. 9 for the solemnity of the Immaculate Conception.