eROI recently released its first quarter 2006 study on email marketing. The results indicate that Tuesday and Sunday are the best days to send email marketing campaigns.
Typically, mid-week has been the best time to send email marketing campaigns as these days typically have higher open rates and click rates. The weekend typically has been the least desirable time to send email. This study, however, reports that Sundays have the highest clickthrough rates and the second highest open rates. Tuesdays see the highest email volume.
If you are contemplating sending an email campaign on Monday, you should probably wait as Monday has the lowest clickthrough rate of any day.