If you are referring to one holiday: Happy Holiday If you are referring to more than one, such as Christmas and New Years: Happy Holidays.
The correct saying is "Happy Holidays", now you know
It is Happy Holidays, with both words capitalized. "Holidays" is pluralized because (generally) you are referring to more than one day of the holiday.
Irving Berlin wrote the song for the 1942 film Holiday Inn, one of my favorite Christmas-time movies ever.
happy holidays
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