APOSTROPHES

Apostrophes can be used: To replace missing letters (e.g., can't, isn't) To show possession (e.g., dog's kennel, dogs' kennel) In temporal expressions (e.g., three months' wages) To show awkward plurals (e.g., 6's and 7's)

When to Use Apostrophes The rules on using apostrophes can be summarised: To replace missing letters aren't isn't can't To show possession Sarah's jumper The dog's toy The dogs' kennel In temporal (or time) expressions 3 months' wages A day's leave 2 days' notice To show the plural of an awkward abbreviation, letter or number BUY THREE CD'S. Mind your p's and q's. Your 5's look like 6's. Unless it is covered by one of the categories above, an apostrophe should not be used. The Most Common Mistake with Apostrophes Words that end in the letter s seem to attract apostrophes. The plural of words ending in vowels (e.g., video, patio) are extremely prone to this error. Anteater's prefer termite's to ant's.    Pearl's melt in vinegar.    Hire two video's for the price of three.   [correct the example] (Do not add an apostrophe just because a word ends in 's'.) Putting an apostrophe before the s in a plural word is a very common mistake and is considered a grammatical howler.

SOURCES : http://www.grammar-monster.com/lessons/apostrophes_using_apostrophes.htm