Simple.


format as date and show only day of week. Then:

If('mycell'=Monday,"M")

And string seven IF statements together there to convert the full
printed day of week out as only a single letter.

For a workbook with a date field that comes close see these:

http://office.microsoft.com/en-us/templates/CT010117277.aspx#ai:TC030008309|

or

http://tinyurl.com/25kboja