Here's some detailed info describing the event:
The Electronic Entertainment Expo, commonly known as E3, is an annual trade show for the computer and video games industry presented by the Entertainment Software Association (ESA). It is used by many video game developers to show off their upcoming games and game-related hardware. The new E3 Expo is run by International Data Group (IDG  ) instead of by ESA.
E3 is invitation-only since 2007, reducing the number of attendees from 60,000 at E3 2006 to the 3,000-5,000 of E3 2007. Starting in 2009, access is restricted to only retail employees with purchasing authority for the organization. A separate conference called Entertainment for All Expo has been created to accommodate the public demand for a major, annual video game event, however it has not been able to replicate E3's success.
E3 was previously held in the third week of May of each year at the Los Angeles Convention Center (LACC) in Los Angeles. In 2007, the convention was exceptionally held from July 11 to July 13 in Santa Monica, California. The ESA stated that the event reached a record attendance of 70,000 people in 2005.
E3 is widely regarded as the world's largest regular convention for video games. Video game companies generally spend more on their presentations for E3 than any other convention (including fancy decorations and pyrotechnics). Major video game critics often have a "best of E3" award session (similar to end-of-year award sessions), but only E3 consistently features such awards (for example, there is rarely a "Best of TGS" award session).
E3 2009 will remain at the LACC June 2nd-4th.