Here is a statement from White House Press Secretary Jay Carney:
"Today, the President asked to address the Congress about the need for urgent action on the economic situation facing the American people as soon as Congress returned from recess. Both Houses will be back in session after their August recess on Wednesday, September 7th, so that was the date that was requested. We consulted with the Speaker about that date before the letter was released, but he determined Thursday would work better. The President is focused on the urgent need to create jobs and grow our economy, so he welcomes the opportunity to address a Joint Session of Congress on Thursday, September 8th and challenge our nation's leaders to start focusing 100% of their attention on doing whatever they can to help the American people."
Originally, the White House wanted a speech on Wednesday night, at the exact same time as a previously scheduled GOP Presidential debate in California, a request that aggravated Republicans in the Congress.