=CHOOSE(WEEKDAY(B2;1);"Su";"M";"Tu";"W";"Th";"F";" Sa")

Gets the prize

Although
=MID("SMTWTFS",WEEKDAY(A1),1)
was good it could not handle the 2nd letter thing ie Sa

If you wanted to use MID with 1 and 2 letter abbreviations, you could do it
this way (note the strategically located spaces)...

=TRIM(MID("SuM TuW ThF Sa",2*WEEKDAY(A1)-1,2))

Rick Rothstein (MVP - Excel)