If I write the word yesterday's mail, should it be yesterdays mail or yesterday'

Question:

If I write the word yesterday's mail, should it be yesterdays mail or yesterday's mail?

Best Answer - Chosen by Asker:

Sorry, but the previous writer is partly wrong and partly right.

Apostrophes are used to designate possession. they are also used to show 'contractions'. For ex: /do not/ becomes don't. And /can not/ becomes can't.

Now for your qu about the mail: today's mail, or tomorrow's mail or yesterday's mail. All have the apostrophe before the /s/ because there is only one mail.

If you had two people's mail in your hand, you would say, "I have yesterdays' mails." The apostrophe comes AFTER the /s/ to indicate that more than one mail exists.

Ditto: 4 cats have 4 black tails
"Look at those cats! The cats' tails are black."

But if only one cat: "That cat's tail is black."

Bit confusing, but follows the rules of English grammar.