We said earlier that a short prepositional phrase at the beginning of a sentence that is just a simple modifier does NOT take a comma. Let's look at what prepositional phrases do take a comma.
RULE: Put a comma after a "long" prepositional phrase. (Though there is no set number of words to necessarily count, the dividing line is somewhere around five words.)
...In the very late afternoon that day, we were...
...During the intercontinental missile crisis, the U.S. was...
RULE: Put a comma after two prepositional phrases. (There are times when these might be modifying separately and be punctuated differently, but we will deal with that at another time.)
...On the day of the accident, I left...
...In the light of day, we could see...
RULE: Put a comma after a prepositional phrase that has a comma inside it.
...On many, many occasions, she was not...
...At the very, very least, he needs...
Stay tuned for more.
Happy punctuating.
Margie