Beginning of the Sentence
If you must use a date at the beginning of the sentence, the month should come first, spelled out. When at all possible, rewrite the sentence to place the date within the text. For example, "September 21 was..." rather than "21 September was..."
When you are writing a date inside the body of text, you can use one of two formats. The U.S. format is "month day, year." The universal or European format is "day month year." The year should be four digits. No comma is needed if only the month and year are provided. Be consistent throughout your document with whichever format you decide to use; don’t switch between the two.
U.S. format: "On February 5, 2000, the…"
Universal or European format: "It went into effect on 5 February 2000."
Dates in References
The dates you use in the references section are spelled out a bit differently than in-text dates. Abbreviate the month to three letters, except May, June and July, which you would spell out, and September, which you shorten to Sept. Just like in-text dates, you should use a four-digit year. For example "Sept. 2012" or "21 Jan. 1998."