Using calendar built-in library, you can handle it like this:
import calendar
s_day = raw_input()
days = list(calendar.day_name)
# days.index('Saturday') will print 5
print days.index(s_day)
Like this, the raw input must be in the correct format. Such as 'Monday'
not 'MOnday'
solved Python – How to determine number of day of week given the Weekday [closed]