We’re under the New Covenant. Leviticus was pretty much a set of commands to the Jews under the Old Covenant, not a set of commands both to Jews and Gentiles. Jews don’t follow any of the sacrifice portions of the Law anymore, so why are we expected to follow Yom Kippur? Of course, some of the old Law still applies, because it’s natural law, or because God reiterates it later through the NT and the Church. But the New Covenant not being under the whole of the old Law is kinda part of the point of Christianity. None of the Law goes away entirely; but that doesn’t mean it applies to us.
Paul goes extensively into what stuff falls under the New Covenant and what under the Old Testament. (For example, a lot of chunks of Corinthians and other letters are organized according to Leviticus’ topic lists, I hear.) Later on, you also get stuff about people going too far with keeping Jewish tradition and becoming Judaizers. That also helps draw the line.
Now, if I were in Israel, and the Israeli government says that Yom Kippur is a state holiday and my Israeli company shouldn’t make me work that day, then I would come under that part of Jewish law as “the stranger who sojourns among you.” Similarly, if I were working for a Jewish company anywhere in the world.
But otherwise, it falls under the whole thing about celebrating festivals and moon days and such, where Paul said it really didn’t matter, or that they should do what the Church in town was doing.
I will add that Yom Kippur really doesn’t make sense for Catholics, because we have Confession at any time, not just once a year, and our sacrifice is Jesus Christ, who died once for all.
The St Paul Center for Biblical Theology has a very nice set of audio talks about which Jewish festivals correspond to Catholic feasts and to events in Holy Week and in Jesus’ life. You might find that useful.
