Well, not quite. It's a timer based service and Google being Americans they primly describe your state on a Friday night as being "Tired", if Google understands irony it might be a reference to Private Eye's "Tired and emotional" euphemism.
Mail you send over the weekend late at night may be useful but you may regret it the next morning.
How it works is you install it through the "Labs" tab on your Gmail settings page, then before you can send an email at certain designated times you have to solve some basic arithmetic problems. Presumably you pre-set the times it's active to match your likely times for being tired and emotional.
The feature is also available for the email component of the Google Applications for Domains hosted office service.
I can see this as a great service for those who go out partying then put a hand over one eye so they can do a quick email check while they are waiting for the room to stop spinning, now I just wish the car manufacturers would install something like this to reduce the carnage on our roads by stopping people driving cars drunk.