How should Sunday be sanctified?
The Third Commandment explicitly forbids servile work on Sundays. Yet, the Church further commands that all Sundays – and all other Holy Days of Obligation – are mandatory days of Mass attendance. Missing Mass on one of these days without a grave reason – illness, inability to reasonably obtain transportation, et cetera – is therefore a mortal sin. And, if one were not able to attend the Holy Sacrifice of the Mass for a good reason, one should still read the Missal for that day and pray the prayers from the Liturgy (e.g. Collect, Gradual, Communion).
Yet Sunday is also a day in which to participate in communal Rosary, Vespers, and Benediction services. Sunday is the day on which the Faithful should be most willing to read Catholic newspapers, books, and magazines.
What are the Holy Days of Obligation?
Before the mid-17th century, individual bishops could pick the holy days of obligation, so in an attempt to establish uniformity, Pope Urban VIII reduced the number of holy days of obligation (not including Sundays) to 36 total days. In 1911, Pope St. Pius X reduced the number to 8 and in 1917, the Code of Canon Law (1917) increased the number to the ten days still observed.
§1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation. The following days must also be observed: the Nativity of our Lord Jesus Christ, the Epiphany, the Ascension, the Body and Blood of Christ, Holy Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, Saint Peter and Saint Paul the Apostles, and All Saints.
§2. With the prior approval of the Apostolic See, however, the conference of bishops can suppress some of the holy days of obligation or transfer them to a Sunday.
However, the United States Conference of Catholic Bishops moved the Epiphany and the Feast of the Body and Blood of our Lord to Sundays, reducing the number to six days: Ascension, Assumption, All Saints, Immaculate Conception, and Christmas. Yet, many dioceses still move Ascension Thursday to the following Sunday, which does not coincidence with Traditionalist Catholics who observe Ascensiontide, leading up to Pentecost Sunday. Furthermore, in 1998 the U.S. bishops decided that when the solemnities of Mary the Mother of God (January 1), Assumption (August 15), or All Saints (November 1) fell on a Saturday or Monday, the obligation to attend Mass does not remain.
Days of importance in the Church that are not technically classified as Holy Days of Obligation include Ash Wednesday, Holy Thursday, and Good Friday, although the Faithful are still highly encouraged to attend Mass on these days of principal importance in the life of the Church.