Why was it not a good day to launch the shuttle?

The space shuffle Discovery was set to lift off (at 1949 UTC) Saturday from Cape Canaveral, Florida, but thunderstorms moved in, causing the launch to be delayed until another Sunday. NASA is launching Discovery on a 12-day mission to the International Space Station, despite continuing safety concerns.
    Discovery’s seven crew members, dressed in bright orange spacesuits, waved and gave a cheerful thumbs-up-sign as they headed to the hunch pad. The astronauts were then strapped into the space shuffle, which was fueled and ready for take-off. But storm clouds lingered in the launch zone, prompting NASA officials to stop the countdown just minutes before the scheduled launch.
    Launch Director Mike Leinbach announced the delay. "We’re not going to make it today. So I appreciate your support, both from the crew and the whole launch team and the team worldwide for trying to get this vehicle off the ground today, but it’s not a good day to launch the shuffle, so we’re going to try again tomorrow", he said.
    The delay was a disappointment for the U.S. space agency, which last launched the shuffle in July 2005, and is eager to resume flights to the International Space Station.
    Vice President Dick Cheney and several members of Congress were at the Kennedy Space Center as guests to watch the launch.
    Astronaut Steven Lindsey commands a crew of five American astronauts, and will bring German astronaut, Thomas Reiter, to expand the space station crew to three. The team will also deliver supplies to keep the space station running, and two crewmen will practice inspection and repair techniques during a spacewalk.
    NASA says if Discovery’s mission is successful, it will launch another shuttle next month to resume space station construction for the first time since Columbia’s demise.