Don't have a lot of time, but this should be real close at least:

SELECT *,
  CASE WHEN EndDayNumber >= StartDayNumber THEN EndDayNumber - StartDayNumber
   ELSE EndDayNumber + 7 - StartDayNumber END AS DaysDiff
FROM (
  VALUES(1, 'Monday', 'Friday'),(2, 'Tuesday', 'Wednesday'),(3, 'Friday', 'Monday')
) AS test_data(D, StartDay, EndDay)
CROSS APPLY (
  SELECT CEILING(CHARINDEX(EndDay, 'Monday Tuesday WednesdayThursday Friday Saturday Sunday') / 9.0) AS EndDayNumber,
   CEILING(CHARINDEX(StartDay, 'Monday Tuesday WednesdayThursday Friday Saturday Sunday') / 9.0) AS StartDayNumber
) AS calc1