Got a question for you, sister. We just moved to the states from Canada and expected to have Jan. 1st as a Holy Day of Obligation. Well, we were surprised to find out our bishop of Salt Lake City, Utah "abrogated the requirement as a holy day of obligation". We thought that was weird. My sister, who has recently come back into the Catholic Church after many, many years and is struggling with accepting some of the teachings of the church questioned me as to how it could be this way. Where she lives in California, it is a H.D.o.O. but not where I live? She said, "So. If I don't go to Mass it is a mortal sin, but not if you don't go?" I stumbled over that one. It is bothering me. I didn't know the answer for her. I just told her that the Church was divinely instituted but humanly run and humans can make mistakes. (Which is what I think our bishop did - and went to Mass anyways this morning.)
I think you might be mistaken about Jan. 1st being abrogated by the Bishop. But let's back this truck up and see just what goes on with Holy Days of Obligation altogether. That beeping sound you hear is us backing up.
Holy Days of Obligation are special days set aside by the Church that are to be treated like a Sunday. Mass is the "obligation" part and you're supposed to take the day off and put your feet up or putter in the garden or read a good book.
In Europe, there were no less than 34 Holy Days of Obligation throughout the Liturgical year (which begins with Advent). Eventually, they were whittled down to 11. But when all the immigrants flooded into the United States in the 18th century, it was very difficult for Catholics to make it to all 11 due to persecution and the distance folks had to travel to get to church and such. So in the US, the Holy Days of Obligation were whittled down to 10 and then to 6. Two were moved to a Sunday, when you're in church anyhow and two were removed altogether.
Jan. 1st Solemnity of Mary, the Mother of God
The Ascension of Jesus into Heaven
Aug. 15th The Assumption of Our Lady into Heaven
Nov.1st All Saints Day
Dec. 8th The Solemnity of the Immaculate Conception of Mary
Dec. 25th Christmas
As simple as this seems, it's not so simple. The Ascension, for one, is not on the same day every year as it lands 40 days after Easter, which is a movable feast. On top of that, in many places, it has been moved to whatever Sunday is after the 40 day mark.
Which also happens with any other Holy Day of Obligation that lands on a Saturday or a Monday.
So my question to you is: did the Bishop of Utah really abrogate Jan. 1st for this year? He has done it in other years because it landed on a Monday or a Saturday, but you may have read an old posting of some sort because, since it's today, it should still be a Holy Day of Obligation.
Did you commit a mortal sin by not going to Mass? No. Will your sister commit a mortal sin by not going to Mass? Yes.
Which brings us to your initial question: How on earth does that work? In a word, "obedience". We have to follow the laws of the Church and the Bishop and the Cardinals and the Pope decide what those laws are. Like the meat of Friday thing. Many people were told if they ate meat on Friday they would go to Hell. (I was never told that. I was told that was a venial sin, not a mortal sin, but I am in a minority.) It would be a fine how do you do to find yourself in Hell for eating meat on Friday and then get the news that that's not even any kind of sin anymore. I don't worry about it. God can do whatever He wants. The Bishops and the Pope and the Cardinals determine what that is.
They don't just make things up willy nilly. They have your soul's best interest at heart and fuss and argue and read and pray and ponder to figure out the best way to drag you to Heaven. It's their whole job. It's my whole job.
I'm glad you gave yourself and the Bishop the benefit of the doubt and hopped off to Mass. We did that and had one of our favorite dinners, sauerkraut and perogi mixed up together. We'll soon be fat as toads. Good thing Lent is up next.