If the shuttle fails to lift-off in July, NASA will have to wait for the next launch window, September 9 to 24.
The delay has been a major disappointment for the agency, which is seeking to recover its prestige after Columbia broke up as it re-entered Earth's atmosphere on February 1, 2003.
Following an independent probe, the agency was forced to rethink the shuttle program and has made changes to the shuttles to reduce the risk of a new accident.
What is a launch window? Launching a shuttle is a complicated scheduling puzzle. It has to consider weather, daylight hours, the rotation of the earth, flying over populated areas, and lots of things I can't spell.