in dates.
in greetings and closings of letters.
in addresses
and quotation marks in dialogue.
Use punctuation to separate items in a series.
To separate an introductory element from the rest of the sentence.
To set off the words yes and no (e.g., Yes, thank you), to set off a tag question from the rest of The sentence (e.g., It’s true, isn’t it?), and to indicate direct address (e.g., Is that you, Steve?).
Use a comma to separate coordinate adjectives (e.g., It was a fascinating, enjoyable movie but not she wore a new blue dress).
Use punctuation (comma, ellipsis, dash) to indicate a pause or break.