In the United States, dates are expressed using a month-day-year format. In this style, a pair of commas sets off the year from the rest of the sentence:
On September 1, 1998, I arrived in Los Angeles for the first time.
The March 27, 2009, interview yielded unexpected opportunities months later.
In the latter example, the date modifies interview. This construction can be awkward, as the adjectival phrase March 27, 2009, points the reader forward toward interview while the commas around 2009 point the reader back toward March 27. The sentence can easily be rewritten, however, to avoid the issue altogether:
The interview on March 27, 2009, yielded unexpected opportunities months later.
In other parts of the English-speaking world, dates are expressed in the much more sensible day-month-year format in which no commas are used:
I graduated from my master’s program on 15 May 2009.
Also, when only a month and year or a specific day (such as a holiday) are mentioned, no commas are used:
Both my parents and my sister and brother-in-law celebrate milestone anniversaries in June 2010.
My good friend’s baby was born on Mother’s Day 2009.
Do you have a question about the comma? Let me know, and I’ll include it in a future installment of Mots Justes’ ongoing series.
The Mots Justes Series on Commas
Chicago Manual of Style, The. 15th ed. Chicago: The University of Chicago Press, 2003.
Hacker, Diana, The Bedford Handbook for Writers, 3rd ed. Boston: St. Martin’s Press: 1991.