I just found this thread - sorry it took so long for someone to respond to it.
The 1983 Code of Canon Law identifies Sunday as "the primordial
holy day of obligation" in the universal Church. In addition to Sunday there are 10 other days of obligation:
1. The Nativity of the Lord Jesus Christ (Christmas)
2. The Epiphany (Jan. 6.)
3. The Ascension (40 days after Easter)
4. The Body and Blood of Christ (Corpus Christi)
5. Holy Mary, the Mother of God (octave day of Christmas, Jan. 1)
6. The Immaculate Conception (Dec. 8)
7. The Assumption (Aug. 15)
8. St. Joseph (March 19)
9. Ss. Peter and Paul (June 29)
10. All Saints (Nov. 1)
The 1983 Code of Canon Law also states that the conference of Bishops
(usually the bishops of a particular country of a region) can suppress or
transfer the observance of these days, after approval from the Apostolic
See (i.e. the Vatican)
In the United States six of the universal holy days are observed. They are
#1, 3, 5, 6, 7, & 10 from above.
As for the other four, Epiphany is observed on Sunday, so is Corpus
Christi. However, St. Joseph and Ss. Peter and Paul are not observed as a
day of obligation for participating in Mass.
Expanation of the days:
1. The Nativity of the Lord Jesus Christ (Christmas)
This one I hope is obvious.
2. The Epiphany (Jan. 6.)
The holy day celebrates the manifestation of the divine nature of Jesus to the Gentiles as represented by the Magi bringing gifts to the King.
3. The Ascension (40 days after Easter)
The holy day celebrates the Ascension of Jesus into heaven after His resurrection.
4. The Body and Blood of Christ (Corpus Christi)
This feast is celebrated in the Latin Church on the Thursday after Trinity Sunday to solemnly commemorate the institution of the Holy Eucharist. This was instituted by Jesus Himself through a vision to a nun who worked to get it accepted by the Church.
5. Holy Mary, the Mother of God (octave day of Christmas, Jan. 1)
This teaching of the Church reiterates the unity of Christ's two natures: he is both God and man. The Son existed before all time, as the eternally begotten of the Father.
6. The Immaculate Conception (Dec. 8)
This holy day celebrates the miracle of how the Mother of the Lord was preserved from her birth from original sin by the power of God.
7. The Assumption (Aug. 15)
This holy day celebrates that at the end of her life, the Blessed Mother was assumed, body and soul into Heaven.
8. St. Joseph (March 19)
This holy day celebrates the pious life of the husband of Mary the mother of Jesus.
9. Ss. Peter and Paul (June 29)
This holy days celebrates the work of Peter and Paul in expanding Christs Church on earth.
10. All Saints (Nov. 1)
This holy day celebrates ALL saints (those identified by the Church AND those who have not been officially identified).
I hope that is clear and there is enough detail for those who are interested. If not just ask for more.