Compressed days of the week
Retina, 152 88
Massively golfed with @Martin's and @randomra's help!  Thanks both of you!
^
SuMoTuWeThFrSa
([A-Z].)(?!.*\1)

T`l``Su\B|\BSa|o|r|u?We.?.?|uTh
^MTWTF$
D
SS
E
.{7}
A

Try it online.  A couple of the lines start with m`  with this online interpreter link.  This is so the program works with multiple input lines (to run all tests in one shot).  However, multiple input lines is not a requirement, so these are not included above or in my score.