What fundamental need do holiday celebrations provide?

The details may vary from one culture to another. But most if not all human cultures include some notion of celebration.
Celebrating anniversaries of historic events helps integrate the memory of those events into the culture that celebrates / observes them.
Is there more to it than that?

Happy Holiday !

sear said:

"Happy Holiday !" s #1 Dec. 2021

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Hal·low·een also Hal·low·e'en (hăl′ə-wēn, hŏl′-)
n.
October 31, celebrated in the United States, Canada, and the British Isles by children going door to door while wearing costumes and asking for treats and playing pranks.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.

Etymology: short for All Hallow Even (All Saints' Eve)

B.F. Skinner observed that many technological innovations proliferate to solve problems caused by previous technological innovations that proliferated to solve problems.

Halloween?
Some persons believe we die, and it's over.
Some others harbor dim view of such finality, perhaps the fuel for the proliferation of afterlife myths.

Those can be divided into the obvious binary categories:
- life & death
- good & evil
- Heaven & Hell
Do these notions really assuage the alarm of finite mortality?

More directly, has Halloween's satirical view of death helped to diminish alarm created by the myths of post-death sentience?

While we mull that one over ... HAPPY HALLOWEEN !