It's amazing how often this question gets asked. December weather is better than any other, in my opinion. As some have said, it is a dry, desert-like climate, and the temperature can get cold at night. But there will rarely be snow.

The NFR (not NRF) is awesome, better than any of the other sporting events hosted there. They have Cowboy Christmas in the exhibition halls near the Palazzo, and also cowboy shooting contests at the Riv. Here's the 2011 schedule, to avoid or not avoid:

https://www.nfrtickets.com/