Holy Day of Obligation

In the Catholic Church, holy days of obligation (also called holydays, holidays, or days of obligation) are days on which the faithful are obliged to attend Mass.

The obligation is attached to the holy day, even if transferred, as sometimes happens in the Roman Rite, to another date because of coinciding with a higher-ranking celebration. However, in some countries a dispensation is granted in such circumstances.

The holy days of obligation for Latin Catholics are indicated in canon 1246 of the 1983 Code of Canon Law:

Can. 1246. §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.

Placed in the order of the civil calendar, the ten days (apart from Sundays) that this canon mentions are:

The number of holy days of obligation was once much greater. With the motu proprio Supremi disciplinae of 2 July 1911, Pope Pius X reduced the number of such non-Sunday holy days from 36 to 8: the above 10 dates (1 January was then the Feast of the Circumcision of Christ) minus the feasts the Body and Blood of Christ, and Saint Joseph. The present list was established in 1917.

In many countries the bishops had obtained, even before the time of Pius X, the Holy See's approval to diminish the number of non-Sunday holy days of obligation, making it far less than 36. Today too, Episcopal Conferences have availed themselves of the authority granted them in law to reduce the number below the ten mentioned above.

...
Wikipedia