To those who care about punctuation, a sentence such as "Thank God its Friday" (without the apostrophe) rouses feelings not only of despair but of violence. The confusion of the possessive "its" (no apostrophe) with the contractive "it's" (with apostrophe) is an unequivocal signal of illiteracy and sets off a Pavlovian "kill" response in the average stickler.
Lynne Truss, Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation
apostrophe, humor, punctuation, writing