December 8 is the Solemnity of the Immaculate Conception of the Blessed Virgin Mary. However, this year (in 2018), the Immaculate Conception remains a Holy Day of Obligation, even though December 8 falls on a Saturday.

In general, the Immaculate Conception remains a Holy Day of Obligation, even if December 8 naturally falls either on a Saturday or on a Monday. This is because Mary, under the title of the Immaculate Conception, is the Patroness of the United States.

In the time frame, from Friday, December 7, to Sunday, December 9, we are obligated to attend two separate Masses for two distinct obligations: We are obligated to attend one Mass, in order to fulfill our obligation for the Holy Day (The Immaculate Conception), as well as another Mass, in order to fulfill our Sunday obligation (The Second Sunday of Advent).

Since these two obligations are distinct, they must be fulfilled by attending two separate Masses. One cannot fulfill both obligations, at once, by only attending one Mass.