a week would surely be tue,wed,thu,fri,sat,sun,mon tho as i got it monday evening?? and a week is 7 days?
It doesnt have a date on it just says 'a week'
I would personally go back on Tuesday morning, that is the morning of the next working day. As it was Monday night you received the note, I see this as being completely cut and dried.
Were you off work sick on Monday? If so, I'd have thought that you would be expected back on the following Monday, by which time you would have had a week off.