Sundays at 2:27 PST. But you just missed it so you'll have to wait until next week.
Consider for example making a submission to reddit or digg. When would be a good time to do so to maximize the expected number of people who will check it out?
You probably can still do it on Digg - I went after Reddit because I figured it'd be harder, but I bet Digg is full of XSS and XSRF vulnerabilities.
Of course, press releases are probably the most reliable and effective way.