In addition to Sundays, each of the following is a holy day of obligation in the Unites States:
January 1, the Solemnity
of Mary, Mother of God
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 requirement to attend Mass is abrogated.
The following is excerpted from an article at americancatholic.org
Q: How can the U.S. bishops say that we have no duty to attend Mass on a holy day of obligation if it falls on a Monday or a Saturday? The precepts of the Church specify which feasts are holy days, regardless of where they fall in the weekly calendar. We must attend Mass not only on Sunday but also on these holy days. How can the observance of Jesus’ Ascension be transferred to a Sunday?
A: The 1983 Code of Canon Law for the Latin rite specifies 10 holy days of obligation: January 1, Epiphany, St. Joseph, Ascension, Corpus Christi, Sts. Peter and Paul, Assumption, All Saints, Immaculate Conception and Christmas.
That same Code allows Episcopal conferences, with prior approval of the Holy See, to suppress certain holy days of obligation or transfer them to a Sunday (Canon #1246:2).
The Holy See requires that in each country Christmas and one feast of Mary be observed on the actual date as holy days of obligation. The U.S. bishops have designated the Immaculate Conception, our patronal feast on December 8, as our Marian feast.
If it or Christmas falls on a Saturday or a Monday, they are always holy days of obligation in the United States. If January 1, Assumption or All Saints falls on Saturday or Monday, they are not holy days of obligation that year.
On November 16, 1998, the U.S. National Conference of Catholic Bishops approved a resolution that a majority vote of diocesan bishops in each ecclesiastical province would determine if the feast of Ascension is transferred to the following Sunday.
That decision was confirmed by the Holy See. There are 31 Latin-rite ecclesiastical provinces in the United States.
In recent years in the United States, the feasts of Epiphany and Corpus Christi have been transferred to Sunday. The feasts of St. Joseph and Sts. Peter and Paul have not been holy days in the United States for many years.
The precepts of
the Church are interpreted by the Church’s
legitimate authority—in this case, by the Holy See
and the bishops’ conferences.