Market closes for trading, but is still actually moving over the weekend, you can check pre-market trading price on CBNC.
News and fundamentals will blow past you S/R or trendlines without any regards, if this news happens to be over the weekend the market will move accordingly and hence create a gap and if no news at all the market pretty much opens near it’s previous close.
So be careful when leaving a position open over the weekend or be sure to trade to the positive side.
Example: USD/CAD may gap down because of the Data releases that happened today. CAD is strong USD is week. OPEC and others are struggling to revive Oil which is also good for CAD.
So if short on that example, you may want to leave open. But if long, you should close unless that’s not your style of course.