Which rule does the sentence below follow?
An important day in US history is July 4, 1776, and this day is celebrated every year.
Use a comma to separate parts of dates and after years in the middle of a sentence.
Use a comma to separate the parts of a U.S. address [except between the state and the zip code when the address is in a sentence]
Use a comma after a greeting and closing in a personal letter, and after the closing of a business letter.