The Catechism of the Catholic Church explains holy days of obligation this way:

“On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body.

Family needs or important social service can legitimately excuse from the obligation of Sunday rest. The faithful should see to it that legitimate excuses do not lead to habits prejudicial to religion, family life, and health.” [CCC 2185]

Sundays and Holy Days are not obligatory when it is impossible to go (e.g. car breakdown, personal illness, sick child to care for, etc.) In addition, dispensations from Sundays and Holy Days can be obtained from your pastor for a just cause (e.g. work conflicts).

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

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. (via USCCB)