If someone were to ask when is a great time to visit Las Vegas, I would answer late September, since the average daily high drops from around 37C to about 32C and the normal low drops from 23C to 18C. For me, the difference between 37 and 32 is significant, when the temperatures are above 35 I don't want to be out in the afternoon and rush to get things done in the morning, but when the temperature stays in the low 30s I will spread my activities out.
Anyways, those are the norms. This year