It's rare, but sometimes a PGA Tour event has an unscheduled Monday finish. Even more rare, sometimes a PGA Tour event has a Tuesday finish.
PGA Tour events extend beyond Sunday when the weather or other course conditions make it impossible to complete a 72-hole tournament by the planned finish. The PGA Tour wants to complete 72 holes if at all possible, so they'll go to Monday to do so.
The last time a PGA Tour event went to an unplanned Monday finish was the 2018 Barbasol Championship in Kentucky, played opposite the British Open Championship, because of storms. Before then, the 2018 Farmers Insurance Open went to a Monday finish because the playoff between Jason Day and Alex Noren could not be completed before darkness. The same was true of the 2017 Zurich Classic of New Orleans.
Of course, there's been a planned Monday finish for about 20 years on the PGA Tour, with the Dell Technologies Championship ending on Labor Day Monday on purpose. That tournament, which was in its final year in 2018, ran from Friday through Monday.
Even more rare is a Tuesday finish on the PGA Tour. This only happens when the PGA Tour aims for a Monday finish and can't complete play but at least half of the remaining weekend field completes 72 holes on Monday. Then the remainder of the field completes the tournament on Tuesday.
The last time a PGA Tour event went to a Tuesday finish was the 2013 Hyundai Tournament of Champions at Kapalua in Hawaii.