ClickZ writes a great article on when to send out your emails. And of course, it depends which market your in: selling to businesses or selling to consumers.
The best day to send business-to-consumer (B2C) e-mail isn't as easily determined. Preferences peak for receipt on Wednesdays (26 percent) and Fridays (31 percent). Previous reports from eROI suggested Friday to be the best day for sending e-mail. The Wednesday delivery allows consumers to plan their weekends and drives them to shopping experiences. E-mail received on Fridays leads into the weekend and hits consumers when the weekend is top of mind. The low points for receiving B2C e-mail occur on Saturdays and Sundays.
Frequency is another issue marketers face when sending out e-newsletters. The widely accepted frequency of monthly for B2B messages remains with 35 percent of respondents stating their preference. However, 25 percent say they would like a weekly newsletter. Among consumers, monthly is the preference. Thirty-six percent said they want monthly e-mail; 21 percent would like e-mail every other week; and 17 percent like weekly e-mail.
It's possible for frequency or the messaging to push subscribers to unsubscribe. Sixty-five percent of subscribers said they unsubscribe when a newsletter is not relevant. Fifty-six percent unsubscribe to newsletters with too high a frequency. Thirteen percent actually unsubscribe when a newsletter becomes "too relevant."