When we talk about commas, we are talking about several hundred individual rules. It it helpful to remember that commas are used in just two ways: to separate and to set off.
The comma that separates pushes two language elements apart.
...We met on Sunday, January 5.
...It was a gorgeous, beautiful day.
...Before we left, I checked all the doors.
The commas that set off surround an element. All it means when we set an element off in commas is that the element can be lifted out of the sentence and there is a grammatically complete sentence left.
...It is clear, in my opinion. that he was involved.
...I want to know, Your Honor, how you recommend I proceed.
...He left on May 13, 2013, for the Army.
Happy punctuating!
Margie