Christmas day and Holy day of obligation

Christmas day noun - A Christian holiday celebrating the birth of Christ; a quarter day in England, Wales, and Ireland.

Holy day of obligation and Christmas day are semantically related. In some cases you can use "Holy day of obligation" instead a noun phrase "Christmas day".

Holy day of obligation noun - A day when Catholics must attend Mass and refrain from servile work, and Episcopalians must take Communion.

Christmas day and holy day of obligation are semantically related. Sometimes you can replace phrase "Christmas day" with "Holy day of obligation", this phrases are similar.