The three network evening news anchors - Katie Couric, Charles Gibson and Brian Williams - were to announce the plans on the network morning news shows Wednesday, but word slipped out earlier.
"For people struggling with this disease, or those who will be diagnosed, scientific breakthroughs can be a matter of life or death - literally," said Couric, whose husband and sister both died of cancer. "We want everyone to know that they can make a difference in this fight."
Hollywood producer Laura Ziskin, a cancer survivor, will produce the event.
The rare simultaneous broadcast is reminiscent of a benefit for survivors of the Sept. 11 terrorist attacks that was aired on many networks weeks after the event.
