Map from Google - Las_Vegas

In Las Vegas, the largest city of Nevada, the climate is desert, mild in winter, but with cool nights, and very hot in summer. The city is in fact located in the Mojave Desert, the westernmost of the American deserts, in the same area of the Death Valley, compared to which, however, it is less hot, because of altitude.
The city is not in fact located in the plains: in the metropolitan area, which includes cities such as Henderson, Spring Valley and Boulder City, the altitude varies from 500 to 900 metres above sea level; the centre of Las Vegas is at 610 metres.
Here are the average temperatures.
Average temperatures - Las Vegas
Las VegasJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)46101319242726221584
Max (°C)141721263237403934271914

Rainfall is very scarce, given that it amounts to just 100 millimetres per year; the rains, though rare, are more common in winter, when they are linked to sporadic weather disturbances, and in summer, when they occur in the form of showers.
Here is the average precipitation.
Average precipitation - Las Vegas
Las VegasJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)152010533108881015115

Winter, from December to February, is mild and sunny during the day, while nights are cold, with minimum temperatures often around freezing. The rains are very rare. Every now and then, short cold periods may occur, in which the maximum temperature remains about 7/10 °C, and sometimes even below 5 °C. Snowfalls are very rare: they generally occur in the hills around the city, above 1,000 metres. Sometimes, however, snowfall can occur even in the city, as happened in December 2008. Intense frosts are rare, although in January 1937 and 1963 the temperature dropped to -13 °C.

Spring, from March to May, is sunny and gradually warmer; at night it can still be cold in March, and more rarely in April. In some years, already in May there can be the first torrid days, with peaks of 40 °C.

Summer, from June to September, is remarkably hot and sunny, with a relentless sun, but also with a dry wind blowing in gusts. In June, the air humidity is very low, often below 10% during the day; in July it remains low but increases a bit, especially at certain times, when moist winds from the Gulf of California, which may also cause showers and thunderstorms, usually of short duration, arrive here. Sometimes there may even be some days in which the air is a little sultry. In general, however, the sun shines, and especially in July and August the temperature goes easily above 40 °C.
Heat waves can be substantial: for example, in July 2005 the maximum temperature exceeded 45 °C for 8 days in a row, with a peak of 47 °C.

Autumn, in October and November, is sunny and warm at first, and then gradually becomes milder. In November, the nights begin to be a little cold.

The amount of sunshine in Las Vegas is great all year round: the sky is almost always clear, it is in fact one of the sunniest cities in the world.

Las Vegas

To the east of the city, Lake Mead, created in 1935 by the construction of the Hoover Dam on the Colorado River, is located at 370 metres above sea level, and in summer is very hot: in Callville Bay, the maximum temperature is on average 43 °C in July and 42 °C in August, and in the worst moments it can reach 50 °C.
To the north-west of the city, the Spring Mountains, whose highest point is Charleston Peak, 3,633 metres high, receive heavy snowfalls in winter, and are equipped with ski lifts. In Mount Charleston, at 2,300 metres in a valley between the mountains, on average two and a half metres of snow per year fall. In summer, in these mountains during the day it can get hot even above 2,000 metres; sunshine prevails, although thunderstorms can break out in the afternoon.

When to go

The best times to visit Las Vegas are spring and autumn, to avoid the summer heat, and the albeit usually not intense winter cold, and in particular from mid-March to mid-April, and from mid-October to mid-November.
If you want to make excursions to the warmest desert areas, like Death Valley, and maybe even to ski in the Spring Mountains, you can instead choose the period from November to March.