If there is ever a comma that I feel confidently about, it’s the Oxford comma. Although some style books say that it’s not necessary, I think it makes perfect sense.
The Oxford comma (also called the serial comma) is the comma used before a coordinating conjunction in a series:
The flavors were chocolate, strawberry, and vanilla.
If you don’t have a comma there, if you have a more complex series, it becomes more confusing what is part of the series and what is part of a single item:
The flavors were strawberry shortcake, lemon meringue, and peaches and cream.
The flavors were strawberry shortcake, lemon meringue and peaches and cream.
Thus, as someone who constantly has groupings of threes in her paper, I enjoy the Oxford comma a great deal. Sorry, Vampire Weekend.
Ps. My categories and tags are all missing for the moment. I hope to get it fixed soon!