One last comment on this: We have only two basic ways that we use commas. Every comma we use falls into one of two categories:
ONE comma pushes things apart. It is the "separating" comma. It is put in so that two things do not pile up on each other. So we put one comma between the month and the year because we don't want these two to pile up on each other.
...on March 10, 2013.
TWO commas (or maybe some other mark of punctuation but two total) says that we are setting something off. The basic concept here is that an item is surrounded because we could take it out of the sentence and still have a grammatically intact sentence left. That is the bottom line for what a pair of commas means. One of the things we surround is the year when it comes after the month and date.
...on March 10, 2013, when we...
We don't change this when the full date becomes an adjective.
One comma implies that we want to separate "March 10" from "2013 letter."
...We sent the March 10, 2013 letter to your office.
The implication is that "March 10" is one unit that needs to be separated from "2013 letter to your office."
If we surround "2013" and can lift it out, we have a grammatically complete thought left.
...We sent the March 10 letter to our office.
This rule simply follows the way we use commas.
Hope this helps explain the rule.
Happy punctuating!
Margie