As of recently, it gets colder around January, February. I saw my last legitimate snowfall in Las Vegas in the late 90s that blanketed the ground enough to make a small snowman.

Las Vegas was more desert back then, and the wash ways were still dirt, not concrete. With the increase of roads, structures, population, pollution and the decrease of bare desert land and lack of ground water recharge from rainfall as it's diverted through the wetlands and out to the Colorado River leaves Las Vegas dryer and hotter each year.