After writing a school essay, I started to think about what is the most accurate/"correct"/used way to indicate dates in written language. Knowing that there are many different alternatives to choose from, it really started to bother me, which of them should I actually use. And what is most important, which of them is so accurate that even my teacher wouldn't have anything to complain about.

An example:

May the 16th, 2011 (I think this is the form we have been taught to use. Is it really a must to use the definite article here (especially in written English)?)

And how about these quite similar alternatives?

May 16th, 2011 (I think I used this one. Is this form informal or even incorrect to use in written English?)

May 16, 2011 (I have noticed that this form is very commonly used, but is it more incorrect than the two previous ones?

Thanks in advance!
It's very difficult to make a mistake with dates. Everything you suggest is used by native speakers. Quite a few would prefer a comma after the year 2011 if the sentence continues.

Your dates are fine.

The formatting of dates is usually dictated by the publisher's "Style Guide." That is so there will be consistency among all the writers who contribute to the publication.

For example, Wiki has the following style guide: http://en.wikipedia.org/wiki/Wikipedia:Manual of_Style(dates_and_numbers)

and the University of Texas has this style guide: