Use a comma:

1. to set off a prepositional phrase (or two) at the beginning of a sentence, especially if it stretches beyond three or four words, or might confuse our reader:

In the majority of these California adoption cases, the state has to show . . . .

2. at the end of a conditional clause (See Tips: “Protasis Up the Apodosis”) and ("What Position My Condition Is In"):

When a party invokes the privilege, there is a conflict . . . .

3. when separating two independent clauses joined by a conjunction:

The comments are disingenuous, and they must be dismissed.

4. before the final and in a series (See Tip: “The Answer Is, ‘Yes, always!’”):

SeaPro provides services for logistics, fishing and drilling, and leasing and provisioning vessels.

5. before a participle introducing a dependent clause:

Parc waved to the crowd, shouting that . . . .

6. for parenthetical asides:

. . . which, except for the date, will remain the same.

7. after the year in a date (see The Chicago Manual of Style):

Tickets went on sale May 31, 2016, and in two minutes . . . .

Sometimes, we use commas merely to encourage our reader to read our words the way we want them read. Nothing wrong with that. But mostly, we use them so we do not confuse our reader.

You may go back to sleep now.