This year the solemnity of the Immaculate Conception is transferred to Monday, Dec. 9
As to not conflict with the second Sunday of Advent. In prior years, when the Solemnity of the Immaculate Conception was transferred to a Monday, the obligation to attend Mass was waived.
The Vatican has since offered clarification to the U.S. Bishops that even when the Solemnity falls on a Monday the obligation to attend Mass remains.
Evening Masses on Sunday, Dec. 8, may be used to fulfill the Dec. 9 holy day obligation provided that you also attend a separate Mass to fulfill your Sunday obligation.
We look forward to seeing you when we celebrate this holy day together on Monday, Dec. 9 this year!