On Mon, Dec. 21, 1986, we met in St. Paul, Minn.
Here's why:
* Commas: Commas are used to separate the different elements of the date.
* Abbreviations: "Mon" and "Minn" should be abbreviated.
* Periods: Periods are used after abbreviations like "Dec" and "Minn".
* Capitalization: "St. Paul" is a proper noun and should be capitalized.
* Final punctuation: The sentence ends with a period.