Learn English – Holidays or holiday?

You're right that "thanks" is always used as a plural noun, whereas when we say "thank you," the word "thank" is a verb.

You can explain that some nouns are always used in their plural form, and give other examples (clothes, outskirts, glasses, scissors, etc.).

You could alternatively tell your students each expression of thanks is a shortened form of a longer sentence that isn't really in use anymore.

(I) thank you. -- Thank you.

Thanks (be to you). -- Thanks.

or

(I give/feel) thanks. -- Thanks.

Google Books search results that pair 'the holidays' with a singular verb

Some U.S. English speakers seem to base their decision about whether to follow "the holidays" with a singular verb or a plural one on whether they understand the notion of "the holidays" as a single continuous thing that stretches across a poorly defined number of days or weeks, or as a series of discrete, freestanding days.* Other U.S. English speakers follow the inflexible rule of "if the subject looks plural, it takes a plural verb." Google Books search results show some (but not much) evidence of idiomatic U.S. use of "the holidays [singular verb]." Most of the matches that do appear involve either reproduced conversations or prose that is very informal in other ways.

From Mattie Jordan & Elisa Baldwin, Where the Wild Animals Is Plentiful: Diary of an Alabama Fur Trader's Daughter, 1912–1914 (1999):

Thursday [December] 25 [1913]. Merry Christmas to all. The wind is sure whistling through the pines and is sure chilly. ... This is the happiest Christmas I ever spent. For all is able to be up and ready to laugh and talk. Even the holidays does not keep people from bringing furs to Papa. Joe Asposito Brought two possum hides today. We have got several furs hanging around the walls. I sure hoped that we would not have any furs during Christmas. For I sure wanted A few days away from the furs.

In view of this book's title, it is hardly surprising that "the holidays" is linked to a singular verb. The writer consistently uses singular verbs with plural subjects, so "the holidays does" is not an instance of "the holidays [singular verb]" as an idiomatic special case; on the other hand, it indicates the existence in the early 1900s of a general pattern of colloquial speech that may have helped make "The holidays is" more common in parts of the United States than in the UK.

From Evangeline Michell, The African American Law School Survival Guide (2006):

One of the biggest mistakes African Americans make when looking for a summer job or that one job after they finish law school is they wait too late to apply. If you want a summer job start applying in January. The government usually selects by March. I know you have a lot going on, it is right after the holidays but the holidays is a good time to put your resume together, research places to apply, call people to get the scoop on who is in charge of hiring and to check the website.

From Melissa Leonard, Gift Giving for Mommies: Empowering Women at All Life Stages (2008):

Save the spending for other holidays such as Christmas or Chanukah. During these holidays, we will have to spend more, but it must work within the budget you have set. The holidays is about being with family and friends, not about who gives what and how much that item cost.

From Markku Kopare, Tales from the Eagle's Nest - a View 2 a Prophecy (2011):

"Well U don't understand!! It's not THAT simple! I've got my job 2 worry about and then the Holidays comes and my kids need this and my wife needs that and bl, bla, bla...."

From Mark Spitz, "Playwright's Notes," in Marshmallow World (2012):

I didn't know Aaron but he was incredibly funny as Ray, the man obsessed with the "suicide chord" that (he believes) drove both Kurt Cobain and Brad Delp of Boston to their early ends. The holidays is a time when you want to be surrounded by family, and when someone in one of our shows dies, it's only ever going to feel like losing a relative somehow.

From Joe Alaska, The Road to Dutch Harbor (2013):

Soon after I gave my two week notice to Larry, Dave the lawyer pointed out to me I had agreed in my contract to give a 4 week notice. It was somewhere in the fine print. Fine. Knowing I was soon leaving made dealing with Minnie much easier. I talked with her only when I had to. At the time, I was set to leave Thanksgiving weekend. However, I decided to stay a while longer to handle the ASRC dividends. I ended up staying seven weeks. Besides, the Holidays was not going to be the best time to look for a new job. I did not want to leave Nunamiut in the middle of chaos.

Google Books search result for 'the playoffs was'

A somewhat similar situation arises in connection with the sporting term playoffs, which refers to a series of contests played after the regular season has ended, to determine that year's league or division champion. Like "the holidays," "the playoffs" is generally understood to be a plural entity and to take a plural verb. But in Google Books search results, there are exceptions where speakers or writers attach a singular verb to "the playoffs." Here are two, both involving baseball playoffs.

From Brent P. Kelley, interview with Frank Seward, in The San Francisco Seals, 1946-1957: Interviews with 25 Former Baseballers (2002):

[Seward speaking:] The second year my record during the year was 18 and 13 and then I won two in the playoffs. So it was considered in those days the playoffs was part of your record, which would make it 20 and 13. That's the year that Larry Jansen, he won 30. I guess Cliff Melton, he probably had 15 or 16 ballgames[.]

From Ken Kaiser & David Fisher, Planet of the Umps: A Baseball Life from Behind the Plate (2003):

In 1984 we went on strike during the playoffs. Only a few umpires were assigned to work the playoff games. So, for most of us the playoffs was the perfect time to strike—we were refusing to work when we weren't scheduled to work anyway. But it was embarrassing to baseball to have scabs working what were the most important games of the season.

Conclusions

Instances in which an author has attached a singular verb to "the holidays" are quite rare in the Google Books database of books and periodicals. But from the tenor of the examples that a Google Books search does find, I imagine that the usage in colloquial speech in the United States is considerably more common. Three of the five recent matches for "the holidays [singular verb]" that I've cited in my answer involve one or another form of "the holidays is a time"—which invites speculation that the singular verb is most likely to occur when the speaker or writer thinks of "the holidays" as a single continues entity, like a season.

Even in the United States, however, the usage is unusual and (from most publishers' point of view, I suspect) highly informal. And I didn't find any instances of the usage in UK sources.

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_{*The sense of "the holidays" evidently differs between North American and UK usage. According to Margery Fee & Janice McAlpine, Guide to Canadian English Usage, (2007):}

_{holiday, vacation, holidays A one-day break from work is a holiday in Canada, the United States, and Britain. Such days may be called 'public', 'civic', 'general', or 'statutory' holidays. When the reference is to a longer break, vacation and holiday(s) are used differently in Britain and North America.}

_{Canadians and Americans use vacation, holiday, or holidays to refer to any extended break from school, work, or regular routine. However, vacation is more often used to refer to the summer break, while the holidays is more often used of the week or so that many people take off between Christmas and New Year's. Similarly, the vacation season usually means summer, while the holiday season usually means the Christmas season.}

_{Fee & McAlpine doesn't define the duration of "the Christmas season," but if its defining features are the appearance of Christmas-themed commercial displays (colored lights, wintry scenes, Christmas trees, etc.) and the playing of Christmas carols at retail outlets, "the Christmas season" seems to start around Halloween (October 31) in many U.S. cities.}