What is the correct punctuation for On Mon Dec 21 1986 we met in St. Paul Minn?

The sentence probably should read:

On Monday, December 21, 1986, we met in St. Paul, Minn. (APA style)

Abbreviations other than for Minnesota would normally not be used, because they add periods within the sentence, but they would make this:

On Mon. Dec. 21, 1986, we met in St. Paul, Minn.

In other styles, the postal abbreviation can be used

On Monday, December 21, 1986, we met in St. Paul, MN. (period is sentence end)