So let’s talk prepositional phrases at the beginning of a sentence.
What if this prepositional phrase is just a simple modifier, a simple adverb modifier?
…On Monday we will begin the new program.
…On April 9 he came in and resigned.
…In the afternoon I had the responsibility to gather them together.
…Before the contract we had to go in individually and talk to the boss.
…At 7:00 he would promptly call me daily.
Notice that these are short prepositional phrases; that they are just modifiers, not parentheticals or conjunctions; and that they have no comma after them.
Rule: A short prepositional phrase that is a simple modifier takes no punctuation after it.
And what constitutes a “short” prepositional phrase? Tune in tomorrow.