For a B2C message, the best days have generally been Friday, Saturday and Sunday late afternoon-early evening.
The provisos being these time choices do not take into account the particulars of your target audience and message. What if you have multiple audiences with different time zones? What type of message are you sending? If the missive is a newsletter or regular communication then being consistent in delivery matters more than which day/time you initially choose.
How to Buzz Wiser:
Look at your website stats and schedule your emails at times when your audience tends to visit your site. Your marketing missives will hit their mailbox while they are online.
Set-up tests by dividing your audience database of recipients that have opened mail into A/B lists and sending your message to A at different day than B. Only test one variable (either hour or day) at a time and compare results to see what works best. You can also test by asking a sample of clients and contacts which day they would prefer to receive the email – as well as what information they would like to receive.
A common mistake made in email marketing is sending out either TOO many emails or sending messages that have little or no relevance for the recipient.
In a McLuhan twist, (I’m showing my age) the message trumps the mechanics of the medium. If your subject line and message engage, educate or enlighten you are likely to have an effective call to action and a successful campaign.