A source in senior hotel management explains: "In the old days it was a deliberate policy, designed to get guests back into the casino early in the evening. These days, it’s less about encouraging you to get back to the tables and more about safety and economics. There’s a greater liability involved in overseeing a pool once it gets dark and it’s not cost-effective to provide lifeguards, lighting, and cocktail service when most of your clientele is off partying somewhere else."
Of course, it still holds true that every Las Vegas casino resort would prefer that you were "off partying somewhere else" -- i.e., and spending money inside the casino -- than lounging by the pool for free. So sunset is when most pools close, around 8 p.m. in the summer months and 5-ish during the winter (for those pools open year-round, of which there are many).
One exception we know of is the Tropicana, which has long been famed for its luxurious pool area. They have three pools altogether, including an indoor-outdoor heated pool that’s not only open all year round, but also 24 hours a day. You have to be a guest of the hotel to use it, however.
