What's with "Friday news dumps"?

By Karl Rove "Every modern White House has put out news on contentious issues late on Friday in the hope that doing so will bury it, or reduce the amount of critical scrutiny it would otherwise receive. What is unusual is the degree to which this White House has relied on this tactic.".... "Friday night network television news and Saturday newspapers and cable coverage are traditionally less seen or read. By Sunday morning, a Friday announcement is often considered old news. Monday is the first opportunity White House correspondents get to ask the president's press secretary on camera about whatever was released Friday. By then there is almost always other news occupying the headlines."