How late are the pools open till at Caesars Palace in May?

Pools at most resorts close in the early evening in Vegas.  They don't want you hanging out at the pool at night - they want you in the bars, clubs, restaurants, shows, and casinos (some of which still aren't open right now).  But late nights at the pool were never really a thing in Vegas, even before COVID and outside of winter, aside from some late-night pool parties that would occasionally be scheduled. 

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MS is correct.

In the summer pools usually close around 6pm.

Pools in many of the resorts closed in winter in Vegas and had early hours until it warms up and the winds die down. Nothing to due to COVID.  You could call and ask if they have a timetable yet for their hours. Pools are usually never open late--that is for the casinos. (and it can get pretty windy too)