We've had the best luck on Thursday night mailings and noticed a bump in our traffic and orders over the weekend as a result. We've also noticed that some people read their email 2 and 3 times. We've used a newsletter format.
The Trick Shop E-Newsletter
Our customers have also said that they liked having our search box included in the email.
I think a lot of it depends on the call to action message in the email though. I really feel like if the promotion or message is strong enough, you can send it anytime.
I hope some other will post their thoughts, too!!