WordNet
n. a day when Catholics must attend Mass and refrain from servile work, and Episcopalians must take Communion
Wikipedia
In the Catholic Church, holy days of obligation (also called holydays, holidays, or days of obligation) are days on which the faithful are obliged to attend Mass.
The obligation is attached to the holy day, even if transferred, as sometimes happens in the Roman Rite, to another date because of coinciding with a higher-ranking celebration. However, in some countries a dispensation is granted in such circumstances.
Usage examples of "holy day of obligation".
So he happened to pass a splendid basilica still intact, and to see in the vicinity some Pavesi who were completing the demolition of a little palace, hard at work even on this holy day of obligation.
Every day seems like a holy day of obligation for the Orthodox from what I've seen.
There were three dozen people in the church, not many for a holy day of obligation.
Every day seems like a holy day of obligation for the Orthodox from what I’.
Devout, she had never missed a Holy Day of Obligation or Feast Day.
It is true that he went to church every Sunday and on every holy day of obligation, but he went as a business investment.