Flight to Vegas – when are the planes the most empty?

My wife and I are going to Vegas in mid-May. We were going to get a flight on a Thursday night. We are both petrified of flying and really don’t want to be on a plane that is compelely full of people. We’re flying on Southwest from Chicago. When are flights usually the least crowded??? Also, how much time do we need to arrive before departure? Thanks!!

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