DETROIT — This year, Catholics around the world will celebrate the feast of Christmas on Monday, Dec. 25.

So, what does that make Dec. 24?According to the Church’s calendar, in years when the feast of the Nativity falls on a Monday, the preceding day is the fourth Sunday of Advent.

That might be straightforward enough, but since many Catholics attend Christmas Mass on Christmas Eve, does this mean a Mass on Dec. 24 satisfies the obligation for both holy days?According to the U.S. bishops, no.

Because the two feasts are separate and distinct — with markedly different spiritualties — Catholics will need to attend separate Masses to satisfy both obligations.

According to a memo from the Archdiocese of Detroit’s Worship Office, “Those Masses scheduled for the Fourth Sunday of Advent will need to honor the focus of this celebration … it is not Christmas. The Worship Commission suggests that the lights not be lit on the trees for this Mass, that the flowers not be placed yet, in order to preserve the anxious waiting for the Christmas celebration."Practically speaking, though, a variety of combinations could fulfill both obligations for the faithful. For instance, attending an evening Mass on Saturday, Dec. 23, and Christmas morning Mass on Dec. 25 would do the trick. Or, the evening of Dec. 23 and Midnight Mass the following day. Or even a morning Mass on Dec. 24 (for the fourth Sunday of Advent) and then an evening or Midnight Mass the same day (in honor of Christmas itself).

A newsletter issued in February by the U.S. Conference of Catholic Bishops’ Committee on Divine Worship noted that a “two-for-one" Mass cannot occur in the very rare circumstances when two of the six holy days of obligation — the feast of the Immaculate Conception or Christmas — fall the day before or after Sunday.

“When consecutive obligations occur on Saturday-Sunday or Sunday-Monday, the faithful must attend Mass twice to fulfill two separate obligations," the committee said.

The reason Catholics might consider the idea of receiving dispensation from a Monday Mass likely stems from the U.S. bishops’ vote in 1991 to lift the obligation to attend Mass on holy days of obligation that fall on Saturdays or Mondays. But that vote was only for three of the six holy days: the feast of Mary, Mother of God, Jan. 1; the feast of the Assumption, Aug. 15; and the feast of All Saints, Nov. 1.

This does not apply to Christmas and the feast of the Immaculate Conception, which is Dec. 8. Most dioceses have transferred observance of the feast of the Ascension from the Thursday 40 days after Easter to the following Sunday.

The divine worship committee also holds out hope that Catholics would want to go to Mass two days in a row, saying: “It would be hoped, of course, that Catholics foster a love for the sacred liturgy and hold a desire to celebrate the holy days as fully as is reasonably possible."The bishops’ committee also has looked ahead to when this will happen again. In the next 12 years, Christmas will fall either on a Saturday or a Monday four times and the feast of the Immaculate Conception will fall on either of those days three times.

Paulist Fr. Larry Rice, director of the University Catholic Center at the University of Texas at Austin, said the big challenge for parishes this year will be decorating for Christmas liturgies, especially parishes with afternoon Masses on Sunday that will only have a few hours to “turn the church over from Advent to Christmas." Another challenge will be getting volunteers to help set up churches for Christmas Eve.

Some parishes are moving up the time of their Sunday Masses on the fourth Sunday of Advent to accommodate the quick turnaround.

— Catholic News Service contributed to this report