"Pick 1," directs a guide printed in the parish bulletin of St. Joseph Church in York, Pennsylvania. The command in the graphic is listed twice, over two columns: The first lists Mass times for the fourth Sunday of Advent, the second lists Christmas Mass times.

The takeaway: No single Mass fulfills both a Catholic's Sunday obligation and the Christmas obligation. Because they are different liturgical days -- even if they overlap on the calendar -- they require attendance at different Masses.

Typically, Mass celebrated at any time on Sunday -- including Sunday evening -- fulfills Catholics' obligation to attend Sunday Mass. Same goes for Saturday evening Masses that anticipate Sunday Mass. Likewise, an evening Mass before a holy day of obligation (such as Christmas) also typically satisfies a Catholic's requirement to attend the holy day Mass.

This year, Christmas Eve is Sunday. So, many Catholics are asking if attending Sunday evening Mass this year can "count" for both.

Canon lawyer Jenna Marie Cooper recently tackled the query in her regular "Question Corner" column for OSV News.

"Because there are two days of obligation -- Sunday and Christmas -- this means that there are two distinct obligations to speak of. Each separate obligation needs to be fulfilled by attending a separate Mass," she wrote in her column, published Dec. 4. "That is, you cannot 'double dip' by attending a Christmas Eve Mass that happens to be on Sunday and have this one Mass fulfill two obligations."

That may seem straightforward, but there's some nuance, Cooper explained.

"Now for the part that can get confusing: Even though you must attend two Masses to fulfill the two obligations, all this means is that you must go to Mass on that calendar day or attend a vigil Mass the evening before. The readings and prayers do not necessarily need to match the day whose obligation you are fulfilling," she wrote. "So, you could go to a Christmas Vigil Mass on Sunday, Dec. 24, and have it count as your Sunday obligation this year; but if you intend for this to fulfill your Sunday obligation, then you must also attend another Mass on Christmas Day to fulfill your obligation for the holy day."

"Of course, if you were to attend a vigil Mass on Saturday for Sunday, and then the Christmas Vigil Mass on Sunday (Christmas Eve) for Christmas Day, then you’ve got it all covered," she said.

A Catholic also could technically attend Mass twice on Sunday, Dec. 24 -- once for the Sunday obligation, and again in the evening for the Christmas obligation.

Cooper notes that when Christmas falls on a Sunday -- as it did last year, and will again in 2033 -- that "Christmas essentially replaces the Sunday liturgically, which means there is only one obligation."

Regarding the meaning and necessity of a Catholic's "Sunday obligation," the Catechism of the Catholic Church states, "On Sundays and other holy days of obligation the faithful are bound to participate in the Mass."

It goes on to say, "The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin."

St. John Paul II expounded on the meaning of Sunday (and, by extension, holy days of obligation) and Catholics' obligation to attend Mass -- which is rooted in the Third Commandment to keep holy the Sabbath -- in the 1988 apostolic letter "Dies Domini" ("The Lord's Day").

He wrote, "When its significance and implications are understood in their entirety, Sunday in a way becomes a synthesis of the Christian life and a condition for living it well. It is clear therefore why the observance of the Lord's Day is so close to the church's heart, and why in the church's discipline it remains a real obligation. Yet more than as a precept, the observance should be seen as a need rising from the depths of Christian life. ... The Eucharist is the full realization of the worship which humanity owes to God, and it cannot be compared to any other religious experience."