The rules are pretty simple: Use an apostrophe to indicate possession (except with possessive its) and
contraction. Don't put apostrophes where they don't belong. Some writers don't like to see a simple s after
a word ending in a vowel; they're compelled to use an apostrophe. But fight that urge. These examples are all correct:
Four Billys in this class
The Williamses next door
Dos and don'ts
The couple said their "I-dos" at the altar.