Commas, periods, quotation marks--it can be hard to remember all of the rules for punctuation, particularly once you are out of school. Fortunately, when it comes to writing out dates, the rules for comma usage are pretty straightforward.
Use a comma to separate numbers. When the day of the month and the year appear side by side, a comma is used to separate them. For example, in "July 21, 2008," a comma separates "21" and "2008."
Use a comma to separate words. When the day of the week and the month appear side by side, a comma is used to separate them: Monday, July 21, 2008.
Use a comma after the date in a sentence. For example, in "On July 21, 2008, the mail arrived later than usual" or "On July 21, the mail arrived later than usual," a comma separates the last element of the date from the rest of the sentence.
Do not use a comma when writing a date in military or European format. For example: 21 July 2008.
As a general rule, no comma is needed to separate number and word elements of a date from one another. Consider writing dates in European format to eliminate the use of commas altogether.