I schedule my posts to go live at 6 a.m. Pacific time, which means my readers in the Central and Eastern time zones are already at work (and studies show people in those 2 time zones comprise 80% of the readership of blogs & social media updates among people in the U.S. and Canada).
Almost 75% of my readers subscribe to my blog via email, so I set my Feedburner feed to distribute the day’s post at around 9 a.m. Pacific Time.
As @blogsbyheather said, stats-tracking sites will provide you with excellent info about the time of day and days of the week most people visit your blog. That time will vary, depending on what industry/niche you write about, and what type of reader you’re targeting. I don’t think there are any hard-and-fast rules about “must post” times or days. Do what works best for you.