A question has been asked regarding how to handle the back-to-back obligations on December 8 and 9. This would be treated the similarly to the back-to-back obligations for the Fourth Sunday of Advent and Christmas that occurred back in December of 2023.
Masses celebrated on the Evening of December 8 are those of the Second Sunday of Advent. Even though a Mass celebrated on Sunday Evening on December 8 will be the Mass of Sunday, it may fulfill the Mass obligation for either Sunday or for the Immaculate Conception but cannot fulfill both obligations simultaneously. The fulfillment of each obligation requires the attendance of a separate Mass either on that day or the evening prior.
Therefore, a member of the faithful can fulfill their obligations by attending any of the following combinations of Masses:
A Mass on Saturday Evening and a Mass on Monday
A Mass on Saturday Evening and a Mass on Sunday Evening
A Mass on Sunday Morning/Early Afternoon and a Mass on Sunday Evening
A Mass on Sunday Morning/Early Afternoon and a Mass on Monday
A Mass on Sunday Evening and a second Mass on Sunday Evening