Traditionally, Memorial Day weekend marked the start of pool season in Las Vegas.
However, with the introduction of the whole "daylife" concept, pioneered by MGM Grand's Wet Republic that debuted in April 2008, pool season now gets underway as early as March, at least for the party crowd, and many pools extend activities into October, well past the former end date of Labor Day weekend.
Opening dates vary by property, but generally, the first pools start opening the first weekend in March; others continue throughout the month, even though mean temperatures tend to max in the high 60s.
But these tend to be targeted at the nightlife crowd, who are there to be seen and rub shoulders with celebrities and beautiful people, as opposed to getting their designer beach attire wet.
Not until April, when daytime temperatures start hitting the high 70s with regularity, is it warm enough to lounge comfortably by the pool and take a dip. In May, it's about 10 degrees hotter, but not yet the 100-plus-degree furnace-type conditions that prevail during the high-summer months.
Most hotel-casinos actually have at least one heated year-round pool, with hours varying seasonally, although they may close briefly for maintenance during the winter months. None are indoors, however, so air temperatures will be a deterrent to all but the most hardcore or hardy swimmers between October (with lows around 50) and April.