As a conference, the U.S. bishops decided, with Vatican approval given July 4, 1992, that when the solemnities of Mary, the Mother of God (January 1), the Assumption (August 15) or All Saints (November 1) fall on a Saturday or Monday, it is not an obligation to attend Mass for these feasts. Although the obligation to attend when these three holy days fall on a Saturday or Monday is abolished, parishes are to continue to observe these Holy Days by scheduling one or more Masses at a convenient time so that people who wish to participate are able to do so.

Solemnity of Mary, January 1
This feast, closely connected to the feast of Christmas, is the most important and oldest of the major feasts of Mary. Mary’s Divine Maternity became a universal feast in 1931. Liturgical reform initiated by Vatican II placed it on January 1 in 1969. Prior to this, this feast celebrated on January 1 was the circumcision of Jesus.

Ascension of Christ, 40 days after Easter
This feast is celebrated the fortieth day after Easter Sunday and commemorates the elevation of Jesus into heaven by his own power in the presence of his disciples. It is narrated in Mark 16:19, Luke 24:51, and in the first chapter of the Acts of the Apostles. In March 2000, the Diocese of Charleston transferred the Feast of the Ascension to the seventh Sunday of Easter, one week before Pentecost Sunday. The Vatican, at the request of the bishops of the United States, granted permission for the date change, giving ecclesiastical provinces in the United States the authority to make the transfer. Observing the Ascension on the seventh Sunday of Easter allows for heightened celebration and an increased opportunity to education people about the meaning of the feast.

Assumption of the Blessed Mother, August 15
This is the principal feast of Mary. It has a double purpose: first, the happy departure of Mary from this life and second, the assumption of her body into heaven. Little for certain is known about the day, year and manner of Mary’s death. The dates assigned for it vary between three and fifteen years after Jesus’ Ascension.

All Saints’ Day, November 1
This feast honors all the saints, known and unknown. This feast was first celebrated on May 13, 610, when Pope Boniface IV proclaimed the day Feast of All Holy Martyrs in Rome. The intent was to honor all martyrs who were not included in local records. In 835, Pope Gregory IV changed the date and name to November 1 and Feast of All Saints. A fall date allowed people to celebrate with food from the fall harvest.

Immaculate Conception of Mary, December 8
The Immaculate Conception of Mary, the Mother of Jesus, is the belief that God preserved Mary from any inclination to sin, the inheritance of original sin passed on to all mankind from Adam and Eve. The feast of the Conception of Mary appeared in the Roman calendar in 1476. After the dogmatic definition by Pope Pius IX in 1854, it became the Feast of the Immaculate Conception.

Christmas, December 25
This feast, one of the two major feast of the liturgical year, celebrates the birth of Jesus.

Article by Father Kenneth Doyle:

Question – If we are a universal church, why are holy day Mas requirements so different? Even in the U.S., most dioceses have transferred Ascension Thursday to a Sunday. Why not all?

Answer – Part of the current state of the law makes sense to me and part does not. Canon law lists 10 holy days of obligation, but (with the permission of the Vatican) bishops’ conferences within a country may suppress some of them or move them to the nearest Sunday. The result is that there is wide variety from nation to nation; many countries, like our own, have six non-Sunday holy days of obligation. Australia and the Netherlands have two. I can appreciate why certain days might be especially celebrated in certain places. In Italy on January 6, the feast of the Epiphany, Mass is obligatory. Italians traditionally celebrate Epiphany with gift-giving, much as we do on Christmas. In Ireland, March 17 marks the feast of St. Patrick, that nation’s patron, and it is a holy day of obligation. Ascension Thursday is a story in itself. Back in the late 1990s, Bishops in the United States took notice that Mass attendance on Ascension Thursday had been dropping for a number of years. Since the feast occurs on the 40th day after Easter, it can fall anywhere from early May to early June, so people don’t have it fixed in their mental calendars. As a result, wishing to highlight the importance of the Ascension, most of the ecclesiastical provinces in the U.S. transferred the celebration of the feast (and the obligation of attending Mass) to the nearest Sunday.