From the USCCB (which determines the holy days of obligation in the US): “Whenever January 1, the solemnity of Mary, Mother of God, or August 15, the solemnity of the Assumption, or November 1, the solemnity of All Saints, falls on a Saturday or on a Monday, the precept to attend Mass is abrogated.”
“the precept to attend Mass is abrogated” means you don’t have to go to Mass.
December 8 is not on that list, apparently because the Immaculate Conception is our Patroness. I never knew that until reading SuscipeMeDomine’s post. (I knew about the patroness part, but not that it was why we have a holy day of obligation for December 8 even if it is on Saturday or Monday. It makes sense when you think about it.)
The Catholic Church tends to have a rather exact way of using words, so she tends to use words that aren’t in the everyday vocabulary of most people.
Welcome! And the vocabulary thing gets easier after a while.