The apostrophe is a punctuation mark used to take the place of eliminated letters in contractions.
Example: “Don’t” is a contraction of the words “do not,” and the apostrophe takes the place of the second “o.”
Apostrophes are also used – and commonly misused – for plurals, possessives and plural possessives.
Here’s a Bad Example I have seen often: Open Saturday’s
The plural of Saturday is Saturdays, so if you are open more than one Saturday and you want to advertise such, please do it without an apostrophe. However, if you are opening Saturday’s mail, the mail belonging to or possessed by Saturday, then the apostrophe to show possession would be correct. The way to show possession is to use: ‘s
Example: Two of their children are Andrew’s, and three are hers. (Note: not her’s)
Exception: If the word ends in the letters S, X or Z, use just the apostrophe without the S to show possession.
Example: Jesus’ twelve disciples.
The same applies to plural possessives, since most plurals end in S.
Example: Jesus’ twelve disciples’ feet were sore.
Exception: The cobbler’s children’s feet were bare.
(Children is a plural that does not end in S.)
OK, to summarize:
Plurals end in: s
Possessives end in: ‘s (unless the word already ends in S or X or Z, then just add ‘ )
Plural possessives end in: s’ (unless the plural does not end in S, then add ‘s )
And if you’re still not sure, ask me.