You do this to ensure it appears in the Saturday news when no one is paying attention. You never sent anything of value out over the weekend with the understanding that it would be a proverbial tree falling in the forest.
It seems the real-time accessibility that social networking offers has broken down the 'weekend wall.' According to a recent ReadWriteWeb article about the findings of Dan Zarella of Hubspot, the weekend is a good time for tweeting, facebooking and emailing (yes, emailing).
The Hubspot crew has been providing us with great social marketing information like this for some time. This week, they gave a great presentation titled "The Science of Timing." Definitely worth a watch. You are sure to gain some valuable insights that may challenge conventional wisdom.