When do you consider, is the best/worst day of the week for a PM to send out an email?

Andrew - Great input, very valuable direction in your comments.

I think something is mixed up here in the discussion. Regular email doesn't mean there is an urgent matter. Urgent matters should be addressed asap, of course. And email is the worst medium as it is asynchronous communication (and it is communication by definition).
When synchronous communication is needed other ways like F2F, a call or instant messaging would be a better choice - whatever is considered appropriate in the culture, company, and team.

Bests!

I try to communicate based on the particular team and then the team members in them. This has worked for just a few people or 50 +. I pay attention to when I believe they read their email, or ask politely first if they have a minute, or I can see them at their desk, a half a world away, using IM lighting up as a reference point or know that they read theirs emails later in the evening or Sunday nights. Generally my aim is to get a response, an effort an action, as quickly as possible. On occasion if I get really pushy I'll email and then ask them via IM to check their email. Doubling up was a regular practice at HP.

I prefer to send out all "context critical" but not "time-sensitive" emails on Monday between 10-11 AM. These are emails that people *need* to read, but might not have an immediate timeframe on having them read it. Things like status updates, and responses to inquiries, etc. This way most people have gotten their Monday morning emails/coffee out of the way and it's before lunch and you can be sure you get the necessary eyeballs on the content of your email.

George -
Good question. I assume you are talking about planned communication that will be distributed to a large(ish) audience, in which they will actually read it. Email is such a limited communication tool due to the amount of emails we all get. So, the essence of the question really speaks to cutting through that clutter and reaching your audience in a meaningful way.
Many organizations are moving away from email because of this huge limiting factor with email. You see more organizations using more social platforms, like Yammer, to create communities where communication can be targeted.
If email is your tool of "choice", creating a routine is important. A weekly/bi-weekly project email that highlights important information creates a routine that people become accustomed to. Since you control the timing, you can do it any day/time of the week. There are pros/cons for everyday. If you take a queue from social media, Thursdays and Fridays 1-3pm tend to provide traction for posts. These seems to speak to a time when people are more open to communication.
Hope this helps. It is a great conversation and there is no one right answer.