The most popular times to visit Las Vegas include New Year's, July 4th and any holidays that have a three-day weekend. Summer is a good time to go for those interested in swimming, while spring and fall have slightly cooler temperatures that make it easier to sightsee.
Since Las Vegas is a tourist destination, the city has events going on at all times and any time can be an enjoyable time to visit. Hotel room rates increase significantly during events and popular travel periods, so the cheapest days to visit are typically weekdays during non-peak periods. Las Vegas has a variety of shows, so people interested in a specific show should check when it occurs before visiting.