The singular form became a standalone idiom in regional dialects. When and How to Use It Properly
The word compliment (from Latin complimentum , meaning “an expression of courtesy”) is key. The phrase uses the singular possessive construction: “Compliment of the Season,” where “the Season” refers metonymically to the winter holiday period (late November through early January). Unlike “Merry Christmas,” which expresses a wish, “Compliment of the Season” performs the act of complimenting—i.e., sending regards or good wishes. It is a truncated form of “I offer you the compliment appropriate to this season.” compliment of the season
"Compliments" represents multiple items of regard, respect, and good wishes. The singular form became a standalone idiom in