From Catholic News Agency:
The Vatican has clarified that Catholics in the United States must still attend Mass on holy days of obligation even when they are transferred to Mondays or Saturdays, correcting a long-standing practice in the U.S. Church.
In its complementary norms, the U.S. Conference of Catholic Bishops (USCCB) stipulates that when certain feast days fall on a Saturday or Monday, the obligation to attend Mass on that day is “abrogated.” Dec. 8 is typically a holy day of obligation celebrating the solemnity of the Immaculate Conception, but this year the feast day lands on a Sunday in Advent. As a result, the USCCB transferred it to Monday, Dec. 9, according to the 2024 USCCB liturgical calendar.