In the following sentence, which phrase should I use? "of his day" or "of his days"?
"He is one of the most eminent linguistic authorities____."
Do these 2 phrases differ in meaning?
"Of his day" suggests a past situation. We don't say "of his days".
Shakespeare was one of the most prolific playwrights of his day.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.