I take my medicine for diabetes three times a week, every other day,
MWF. Sometimes my blood is drawn on the first day, sometimes the
second day, and sometimes on the third day. I have a different
calculation for each of those days, because no matter how you cut it
on Monday I haven't taken medicine for two days, on Wednesday I just
took the medicine the day before, and on Friday, I've been taking it
every other day for the whole week. If they take my blood on Monday it
will be slightly higher than Wednesday, and Friday will be the lowest
of the three. So I have three different calculations to make the
results equivalent, no matter what day they collect my blood.

M glucose = blood glucose - 30 (Beginning of the week calculation
(BoW))
W glucose = blood glucose -5 Middle of the week (MoW)
F glucose = blood glucose + 10 (end of the week (EoW)

My friends want me to run their blood glucose levels through this
"calculator", but they send me their medicine schedules and it's TThS,
or STTh, or sometimes they get mixed up and send me the day with the
gap in the middles, like FMW, and then what day their blood was
collected. So each time I have to 1) figure out the first day of the
week (it could be any day, even Sunday), then 2) apply the right
calculation.

I thought that in Excel there might be a way if I have a row of
schedules and another row of blood collection days, and the list of
three calculations, and determine the right equation to apply (BoW,
MoW, or EoW). For example

Schedule collection day
TThS Th
ST Th