I went to Grand Canyon in July 2008 before attending a summer school in Los Angeles. I planned this trip by reading just as many tips as I could on hiking down Grand Canyon. Anyone you talk to will strongly discourage anyone from hiking down and back in the same day. But, Grand Canyon is one of the most popular park/trail in the United States. Every year, 2 millions visitors go spend some time in the Canyon, either to take pictures or hike all the way down. There are also water station on the Bright Angel trail. In overall, you never feel alone on the trail, which helps in case of emergency.
There is an hotel at the bottom of the Canyon at Phantom Ranch and I called two months in advance to make a reservation. The Phantom Ranch serves dinner, breakfast and pack in lunch but these are quite expensive. So I set out to climb down with enough foods to spend two days in the Canyon. I clearly had overestimated the amount of food necessary and I could have left for 5kgs of foods before leaving. Anyways, if you leave early in the morning you can definitely reach the Phantom Ranch and hike back during the same day if you can avoid hiking during the afternoon. Just don't mention it to the park rangers.
Colorado River at the bottom of Grand Canyon
If you are lucky, it may rain - a blessing during summer given the temperature can go as high as 42 Celsius in July. It was cloudy when I hiked down.
