One, I would simply like to know - if a Holy Day of Obligation happens to fall on a Saturday (as All Saint’s Day does this year), does a Saturday vigil Mass “cover both”, or must one also go to Mass on Friday (Vigil) or Sunday? I ask because it seems unlikely that my brother will be able to go to a Mass on Friday, and I’d like to know what sort of changes we’ll need to make.
Two, does the duty of Sabbath rest apply also to holy days of Obligation, even if those days don’t fall on a Sunday? It would stand to reason that the answer is yes, but I’m not sure.
You don’t say what country you are in. In the US, when a Holy Day of Obligation falls on a Saturday, the obligation is lifted. This is the case for All Saints Day this year. Going to Saturday evening Mass will fulfill your Sunday obligation. You can always go to an earlier Saturday Mass to hear the readings of the day, in this case, the readings for All Saints and also go to Saturday evening Mass.
It is true that (in the U.S.) the obligation is lifted when All Saints Day, Mary Mother of God, and the Assumption fall on a Saturday or Monday. But it is not the case for the Immaculate Conception and Christmas.
Just clarifying for completeness.
With regards to Ireland, I’m not sure. You’d have to check with the Irish Catholic Bishops’ Conference. The only thing I found on their website lists the Holy Days of Obligation but says nothing about them being abrogated if they fall on a Saturday or a Monday:
What I would suggest for your brother is to talk to the pastor. His pastor can help guide him to discern whether or not it’s impossible to attend Mass on Sunday (since you say that he can do so on Saturday), or if he needs a dispensation from the Sunday obligation (which a pastor can give).
As for the Sabbath rest on Solemnities, the answer to your question is “yes.”
There is a moral obligation, however Church law does not define this exactly (certainly not the way our Jewish brethren define the rest). The point is to do what we can to keep the day holy, and to avoid what makes the day profane. We do what we can. Or maybe say “we avoid what we can avoid.”