Q: Can the obligation to attend Mass on holy days be fulfilled before or after the actual day?
Likewise, can you fulfill the obligation to attend Mass on Sunday on some other day?
Christmas fell on a Friday last year. Was there an obligation to attend Mass the following Sunday? Someone told me that attending Mass that Friday already fulfilled the Sunday obligation. I disagreed.
A: The obligation to attend Mass on a holy day is separate from the obligation for Sunday. If Christmas falls on a Saturday, there is one obligation for that holy day and a separate one for the next day.
Attending a late afternoon or evening Mass the day before the holy day or on Saturday fulfills these obligations. There is no window, a couple days beforehand or afterward. Bishops can transfer some holy days to the nearest Sunday, as is the case for the Feast of Ascension in most of the United States.
My answer assumes that the person can attend Mass on these days without great hardship. If a priest is responsible for several parishes or missions, it might be physically impossible to celebrate the Eucharist in each one on every holy day or Sunday. If a person were so sick that he or she could not leave the house, then the obligation would not apply during that illness. But you did not indicate that was the case in the situation that you described.
