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 2021 Northern Trust event, forced to Monday because of storms from a hurricane remnants. Before then, the 2019 Mayakoba Golf Classic went to a Monday finish as a trio of players were unable to finish before sundown on Sunday.
For about 20 years on the PGA Tour, there was a planned Monday finish, 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.
The only tournament on the PGA Tour schedule that can go beyond Tuesday is The Players Championship, which can be extended in perpetuity to reach a full-field finish by the PGA Tour commissioner.