Thursday is the Ascension, the day the Church marks 40 days since Jesus’ resurrection and the day he ascended into heaven (see Acts 1:1–11). But depending on where you live, you may or may not be obligated to attend Mass on that day.

If you live in the ecclesiastical provinces of Boston, Hartford, New York, Omaha, and Philadelphia, tomorrow is a holy day of obligation for you. That means if you live in New York, Connecticut, Rhode Island, Maine, Massachusetts, New Hampshire, Vermont, Nebraska, or Pennsylvania, you have to go to Mass!

If you live in the rest of the country, the celebration of the solemnity of the Ascension is moved to the Seventh Sunday of Easter — which this year is Sunday, May 21.

Why the different days of celebration?

The National Conference of Catholic Bishops (now called the U.S. Conference of Catholic Bishops) stated in a 1991 decree that in addition to Sundays, there are several days to be observed as holy days of obligation: Jan. 1, the Solemnity of Mary, Mother of God; the Thursday of the Sixth Week of Easter, the Ascension; Aug. 15, the Assumption of the Blessed Virgin Mary; Nov. 1, All Saints’ Day; Dec. 8, the Immaculate Conception; and Dec. 25, the Nativity of Our Lord, Jesus Christ.

However, in subsequent action in 1999, the bishops’ conference “in accord with the provisions of canon 1246, §2 of the Code of Canon Law, which states: ‘... the conference of bishops can abolish certain holy days of obligation or transfer them to a Sunday with prior approval of the Apostolic See,’” allowed each ecclesiastical province in the United States to determine whether to move celebration of the Ascension to the Sunday following the Sixth Week of Easter.

Most provinces decided to move the celebration to Sunday; those mentioned earlier, which are mostly in the Northeast, retained the Thursday celebration. Some moved the date in later years, such as the ecclesiastical province of Baltimore, for example, which decided to change the date in 2002. And the province of Newark decided just last year to move the celebration to Sunday.

The decision often sparks some humor on social media around this time of year: