A Holy Day of Obligation is a principal feast day that, in addition to Sundays, Catholics are obliged by church law to participate in the Mass. The catechism of the Catholic Church says, “The Sunday [and Holy Day of Obligation] Eucharist is the foundation and confirmation of all Christian practice.For this reason the faithful are obliged to participate in Eucharist on days of obligation, unless excused for a serious reason [for example, illness or the care of infants or the elderly]…Those who deliberately fail in this obligation commit a grave sin.” (CCC #2181)
From the National Conference of Catholic Bishops of the 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:
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. *Whenever December 8, the solemnity of the Immaculate Conception falls on a Sunday, it is transferred to Monday, December 9 and the obligation to attend Mass is abrogated. For all other days of the week, it remains obligatory.