I am flying out of Zurich at 11:55am on a Saturday morning (August 1st). I know there is are direct trains from Luzern to the airport that take less than an hour and a half. Is this reasonable or should I spend the night in Zurich instead?
Take the train from Luzern on Saturday morning. The first direct train departs Luzern at 7:10 and gets you to the airport an hour later--plenty of time to catch your flight. The direct trains from Luzern stop at Zurich main station in-route, so even if you did sleep over in Zurich you would wind up taking that same train to get to the airport.