In addition to observance of Sunday as the “Lord’s Day”, Catholics are also obliged to keep certain holy days. These are the “holy days of obligation”. The core remains the same, but are modified to conform to local custom. For example, the Catholic bishops of Ireland might make St. Patrick’s day a holy day of obligation.

In the Catholic Church, one commits mortal sin by not attending Mass on any Sunday of the year or on a holy day of obligation. Below is the current decree of promulgation of the National Conference of Catholic Bishops United States of America:

In addition to Sunday, the days to be observed as holy days of obligation in the Latin Rite dioceses of the United States of America, in conformity with canon 1246, are as follows:

January 1, the solemnity of Mary, Mother of God;
Thursday of the Sixth Week of Easter, the solemnity of the Ascension;
August 15, the solemnity of the Assumption of the Blessed Virgin Mary;
November 1, the solemnity of All Saints;
December 8, the solemnity of the Immaculate Conception;
December 25, the solemnity of the Nativity of Our Lord Jesus Christ.

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 precept to attend Mass is abrogated.

This decree of the Conference of Bishops was approved and confirmed by the Apostolic See by a decree of the Congregation for Bishops (Prot. N. 296/84), signed by Bernardin Cardinal Gantin, prefect of the Congregation, and dated July 4, 1992.

So it’s basically six days: 1) Christmas, 2) New Year’s, 3) Ascension, 4) All Saints, 5) Assumption, and 6) Immaculate Conception. Catholic Churches typically offer morning, noon, and after-work Mass on these days so that the faithful can attend Holy Mass. It seems that the Ascension (which always falls on a Thursday) is moved to the following Sunday.