It seems that some people take their grammar seriously.
By the way, the rule of apostrophes in English is simple.
An apostrophe /aPOStrefi/ means something is missing.
A long long looooong time ago when English was German, the genitive case (OF somebody) was signalled with an -es at the end. And because the English are lazier than the Germans, we dropped a letter.
The boy’s classroom. (One boy.)
The boys’ classroom. (Lots of them.)
Hallowe’en because the v of eve is missing. Think Christmas Eve, or New Year’s Eve.
I told you it was easy.