This year, the Solemnity of the Immaculate Conception of the Blessed Virgin Mary falls on the Second Sunday of Advent. Because December 8 falls on a Sunday, the Solemnity is transferred to Monday, December 9 and is a Holy Day of Obligation. Masses for the Immaculate Conception will be celebrated on Monday, December 9th at 8:15 a.m. and 6:00 p.m.
If you attend Mass for the Second Sunday of Advent on Saturday evening or Sunday morning and then attend another Mass on Sunday at 4:00 pm or later, that second Mass may fulfill the obligation for the Immaculate Conception. However, please note that you cannot fulfill two separate obligations with a single Mass.