Las Vegas, where it is

In Las Vegas, the largest city of Nevada, the climate is desert, mild in winter but with cold nights, and very hot in summer. The city is located in the Mojave Desert, the westernmost of the American deserts, in the same area where the Death Valley is located, compared to which, however, it is less hot because of altitude.
In fact, the city is not located at sea level: in the vast metropolitan area, which includes cities such as Henderson, Spring Valley and Boulder City, the altitude ranges from 500 to 900 meters (1,600 to 3,000 feet); the center of Las Vegas is at 610 meters (2,000 feet) above sea level.
Here are the average temperatures.
Las Vegas - Average temperatures
Min (°C)46101319242726221584
Max (°C)141721263237403934271914
Min (°F)394350556675817972594639
Max (°F)57637079909910410293816657

Rainfall is very scarce, in fact, it amounts to only 115 millimeters (4.5 inches) per year. The rains, though rare, are more common in winter, when they are caused by sporadic weather disturbances, and in summer, when they occur in the form of showers.
Here is the average precipitation.
Las Vegas - Average precipitation

Winter, from December to February, is sunny and mild during the day, while nights are cold, with minimum temperatures often around freezing (0 °C or 32 °F). The rains are very rare. Every now and then, brief cold periods can occur, with maximum temperatures remaining around 7/10 °C (45/50 °F) and sometimes even below 5 °C (41 °F). Snowfalls are very rare: they generally occur in the mountains around the city, above 1,000 meters (3,300 feet). Sometimes, however, snow can fall even in the city, as happened in December 2008. Intense frosts are rare, although in January 1937 and in January 1963, the temperature dropped to -13 °C (9 °F).

Spring, from March to May, is sunny and gradually warmer; at night it can still be cold in March, and more rarely in April. During some years, there can be the first scorching days already in May, with peaks around 40 °C (104 °F).

Summer, from June to September, is remarkably hot, with blazing sunshine, but also with a dry wind blowing in gusts. In June, air humidity is very low, often below 10% during the day. In July, it remains low but it increases a bit, especially at certain times, when a moist wind blows from the Gulf of California, which may also cause showers and thunderstorms, usually of short duration. Sometimes, there can even be some days when the air is a little sultry. Generally, however, the sun shines, and the temperature goes easily above 40 °C (104 °F), especially in July and August.
Heat waves can be remarkable: for example, in July 2005, the maximum temperature exceeded 45 °C (113 °F) for 8 days in a row, with a peak of 47 °C (117 °F).

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

Las Vegas

The amount of sunshine in Las Vegas is great all year round: the sky is almost always clear, and with 3,800 hours of sunshine per year, it is one of the sunniest cities in the world.
Las Vegas - Sunshine

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 meters (1,200 feet) above sea level, and in summer it's very hot. In Callville Bay, the average maximum temperature is 43 °C (109 °F) in July and 42 °C (108 °F) in August, but in the worst moments it can reach 50 °C (122 °F).
To the north-west of the city, the Spring Mountains, whose highest point is Charleston Peak, 3,633 meters (11,919 feet) high, receive heavy snowfalls in winter and are equipped with ski lifts. In Mount Charleston, at 2,300 meters (7,500 ft) in a valley between the mountains, 2 and a half meters (8 feet) of snow fall per year on average. In summer, it can get hot in these mountains even above 2,000 meters (6,500 feet), at least during the day. Sunshine prevails, although thunderstorms can break out in the afternoon.

Best Time

The best times to visit Las Vegas are spring and autumn, in order to avoid the summer heat, as well as the winter cold (though it's usually not intense), and in particular, from mid-March to mid-April, and from mid-October to mid-November.
If you want to go on excursions to the hottest desert areas, like the Death Valley, or if you want to go skiing in the Spring Mountains, you can instead choose the period from November to March.