Ask HN: How many days in advance to contact bloggers before launch?

I'm wondering how many days in advance should we contact bloggers, journalists, and start doing PR (press releases, etc..) before a launch? (so they have time to review the service, write the article) Also, what is the best time/day for launch? Most people say Tuesday at 10AM (pacific time). Any advice or experience that you guys have about conducting a successful product launch? Thanks!