Yes, Catholics are obliged to attend Mass for the Solemnity of the Immaculate Conception. If their parish has a vigil Mass on Friday, they may attend that–or they may attend Mass on Saturday in the morning or evening. However they may choose to attend, they are obliged to attend two separate Masses; one for the Immaculate Conception and another on the Second Sunday of Advent. Attempting to attend one Mass for both days does not fulfill their obligation.
Fr. Vincent Serpa, O.P.