‘Holidays’ is not only the plural of ‘holiday’, it could mean especially in British English a period of cessation from work or one of recreation, leisure or vacation. “Happy Holidays” could mean the period from Christmas eve through New Year day. It could also be used as an adverb with the meaning “during every holiday or most holidays.” Use "holiday" for a singular celebrated day, like Christmas, or in the phrase, "I'm on holiday," if you happen to be around British-English speakers to say you will be away from work. Americans would say, "I'm on vacation" instead.

It's just a countable noun. In the West, especially the United States, people refer to the period around Christmas and New Year's as "the holidays". They may even be lumping Thanksgiving in there, as well.

Holidays is the plural word of holiday.