The Catholic Church has established a number of days outside of Sunday that are holy days of obligation, meaning that Catholics are bound to attend Mass.
The Code of Canon Law lists the following days that are celebrated universally, though each bishop has the authority to add or subtract from this list.
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.
This means that the feast of Corpus Christi (also known as the Body and Blood of Christ in English), is normally a holy day of obligation.
At the same time, holy days of obligation are currently governed by the local bishops’ conferences and can vary from country to country, as the Code of Canon Law states.
§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.
Corpus Christi on Sunday
Initially Corpus Christi was celebrated on the Thursday after Trinity Sunday, in accord with the requests of Juliana of Liege, a 13th-century religious woman who promoted this devotion.
In the 20th century it became increasingly difficult to attend Mass on a Thursday in countries where there did not exist a dominant Catholic culture, and the demands of work prevented most people from attending the celebration.
The feast was then moved in many places to the Sunday following Trinity Sunday to make it possible for more of the faithful to participate in the Mass of Corpus Christi. Since Sundays are automatically holy days of obligation, the obligationto attend Mass on Corpus Christi remains.
Corpus Christi continues to be celebrated on a Thursday in places where it is a public holiday, such as many Spanish-speaking countries, or in other countries where Catholicism is a majority.
Be sure to consult your local diocese to see if it is a holy day of obligation and whether it is celebrated on a Thursday or a Sunday.
