The worst time to come to Las Vegas, if measured by crowds, high demand, and high prices, is New Year's Eve. There's barely a close second. Nearly 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 ferocious and hotel-room rates hit the moon. In short, it's one of the biggest New Year's Eve parties in the country and if you're looking for peace and quiet, stay far away.
The two three-day weekends that bookend the summer are also ferocious for crowds and prices: Memorial Day and Labor Day. Expect to compete with upwards of 200,000 other visitors during those. July Fourth, if it falls on a Thursday, Friday, Monday, or Tuesday, can be added to those two long weekends.
Super Bowl weekend can attract upwards of 175,000 visitors.
Conventions crowd the town all year long and the largest of them can be huge. The Automotive Aftermarket Products Expo (with 130,000 attendees) and the Specialty Equipment Market Assocation trade shows (with 120,000) occurred simltaneously in late October 2006. Last January, the Consumer Electronics Show (CES) attracted 150,000 attendees, while the World of Concrete Expo, later in the month, attracted 85,000. The National Association of Broadcasters in mid-April can hit 135,000. More than 100,000 people attend MAGIC, the men's apparel show in late August.