Anna Damiano, Fiction Manuscript Editor at Polished Arrow Editing

Explains how the teachers' day grammatically written.

This actually has more to do with custom than grammar.

In English, we use an apostrophe and an s to show that something belongs to someone (’s). For holidays like this one, we refer to the day as though it belongs to your teacher, or the teachers.

So we have to decide: is it one teacher celebrating or more than one? If it's one teacher, the apostrophe goes between the word and the s: “Teacher’s.”

If it’s more than one teacher, the apostrophe would go between the plural word and the possessive s: “Teachers’s.” But since we can't pronounce that in English, we always drop the last s. It's simply understood. So if it's more than one teacher, it's “Teachers’.”

Most people would probably say that the day belongs to all teachers, which is why “Teachers’ Day” is the generally accepted answer.

It's not universally accepted, though. There is a similar debate with “Mother’s Day” and “Father’s Day.” In this instance it was decided that it's referring to just your mother and just your father. But in instances where someone has more than one mother or father (such as in adoption), they would probably disagree.

So in conclusion, “Teacher’s Day” is not grammatically incorrect, but “Teachers’ Day” is usually the preferred.

If you want to avoid the controversy altogether, you can always just say “Teacher Appreciation Day,” which is another common way of referring to it.

Source: Quora Answer:
https://www.quora.com/Is-Happy-Teachers-Day-not-grammatically-correct