Forex markets are closed at weekends. So if a major event happens on a Saturday which impacts exchange rates significantly, is there a way to quantify how much before waiting until Monday?
The Forex market opens at 5 PM U.S. EST on Sunday night and the futures market opens at 6 PM EST on Sunday. Asia financial markets in general open about 8 PM EST Sunday night but there is pre-market activity.
The markets could open drastically beyond stop-loss orders.
As S Spring mentioned, Forex markets does have limited hours on weekends, if you want to see market activity before Monday.
Other than that, no, you cannot accurately anticipate how prices will react to news when markets are closed. The market price reflects the combined activity of all market participants, and that activity hasn't happened yet.
While individual traders can determine that their own fair price for 1 CAD is now lower because of news X, you can't easily guess how everyone's opinions will be factored in the Monday price. I bet some hedge funds would be very interested if you developed a system to accurately do this.