Tuesday is the most popular day to send out e-mail. Wednesday is a close second and then Thursday. Two recent reports have come out and stated that Monday is becoming the best day to send and the best time of day to send out a mailing is between 6 and 10 am. To determine what best works for you try pre-testing a small group of users, evaluate the statistics and then apply the knowledge from this to future e-mail campaigns.
If It’s Tuesday, Press the "Send" Button, emailLabs, 12/16/2003
Email Deliverability Rates Impacted by Time Campaigns Sent, ReturnPath, July 2004
New Email Stats Reveal Monday is Best Day to Send Campaigns, marketingsherpa, 07/22/2004
eINFO - a collection of articles and studies about email marketing