I'm sure everyone's weight fluctuates from day to day, just like mine - and on Monday mornings you might not be at your lowest weight of the week. If you want to choose your lowest weight during the week prior and use that number as your start for the next week, you may. That way, you will be working harder to get at least 1 pound lower than your LOWEST weight from the last week, versus where you happen to be on Monday morning. Does that make sense?
If you still want to just weigh yourself once a week, on Mondays, that's fine too. Your choice!