Explanation of Westercon from its website:
In 1948, Walter J. “Doc” Dougherty of the Los Angeles Science Fantasy Society proposed that an annual West Coast convention be started for West Coast fans who couldn’t afford to travel east each year. The first Westercon was held in September, 1948. It was a one-day event with an attendance of 77. It wasn’t until 1951 that the Westercon became a weekend-long event, settled into the July 4th holiday and began traveling to different cities. A complete history of the Westercon conventions is available on the Westercon site.
The Westercon Region starts from 104° West Latitude in the east (a smidge between New Mexico’s eastern border and Wyoming’s eastern border, chopping off the east-most portion of Colorado), covers North America westward and includes Hawaii.
More information about Westercon is also available on Wikipedia.