as an adverb, yay means so, this and is accompanied by a hand gesture
as a noun, yay is the name of the y sound. Its plural form is yays.
For US, slang coming from Spanish llello, yay means cocaine.
For Azeri, the noun yay means summer.
For Turkish, the noun yay means bow (weapon)
As I already mentioned, a Yay Day is filled with interjections. If it’s about Interjections, it means that it’s about Grammar. If it’s about Grammar, by definition it means that it’s about my ultimate favorite… Whee! 🙂 Grammarly Handbook
What does Grammarly say about Interjections?
Incomprehensible sounds (MC’s note: such as ‘Yay!’) have a grammatical term for them: interjections.
Interjections are usually found at the beginning of the sentence, as we use the interjection either to get someone’s attention or to give ourselves time to think about the real words.
Interjections are frequently followed by an exclamation mark.
Interjections (and exclamation marks) are generally frowned upon in formal writing.
Interjections are not grammatically connected to any part of the sentence (i.e. they don’t modify a verb or a noun, etc.) They can be removed without creating confusion.
Interjections are used to communicate an extreme emotion which is difficult to verbalize, or to get someone’s attention.
aha, ahem, ahh, ahoy, alas, arrggh, aw
bam, bingo, blah, boo, bravo, brrr
dang, drat, darn, duh
eek, eh, encore, eureka
gadzooks, gee, gee whiz, golly, goodbye, goodness, good grief, gosh