Is it "Father's Day" or "Fathers' Day"?

Yes, I had. But that led me to this post, also in that thread.

So. You’re 16 and you just discovered Bill Bryson, of all people, and now are very full of yourself. (Bryson, you’ll find out if you hang around here longer, is roundly denouncedfor Mother Tongue and every single book he’s ever written as being full of mistakes, idiocies, and general flatulence.)

Read Guy Deutscher. Or David Crystal. Go and dig up an old copy of Jim Quinn’s American Tongue and Cheek. Then go on to more technical works. Your blood pressure will go down and people will stop sidling away from you at parties.

But whatever you do, please, and I mean this sincerely, do not post on the subject of English again while you still believe that the difference in usage between hung and hanged is a matter of grammar.