The worst time to come to Las Vegas if measured by crowds is New Year's Eve. 300,000 people clog the city, with 200,000 of them jamming the Strip and another 25,000 or so packed downtown. The lines for everything are immense and hotel-room rates skyrocket.
The two three day weekends that are on in the beginning and end of summer are also huge for crowds and prices: Memorial Day and Labor Day.
Expect crowds of 200,000 for July Fourth, if it falls on a Thursday, Friday, Monday, or Tuesday, can be added to those two long weekends.
Super Bowl usually attracts crowds of upwards of 175,000 visitors.
Conventions and Tradeshows also can pack Las Vegas. The biggest conventions are as follows:
Automotive Aftermarket Products Expo - 130,000 attendees
Specialty Equipment Market Assocation - 120,000
Consumer Electronics Show (CES) 150,000 -200,00 attendees
Concrete Expo - 85,000 attendees
National Association of Broadcasters 135,000 attendees
MAGIC men's apparel show 100,000 attendees