eROI has just completed their Q3 2007 email study.  Some highlights:

Day of Week:

• Monday and Wednesday lead the way for users to open email.
• The weekends, for the first time in over a year, are the low point.
• Clicks on the other hand see spikes on Wednesday, Thursday and, the highest day of the week, Saturday (however, Saturday volume is so low it is incomparable to the rest of the week).

Time of Day:

• Activity builds as the day moves on with Wednesday afternoon being the most active time for email.

Consumer Industries:

• Retail (brick and mortar) companies bring up the rear with a 19% open rate and a 1.6% click rate. This is far from ideal.
• Travel and Lodging is leading the pack with nearly 50% of all subscribers opening their email and almost 10% click on an email, this is great in anyone’s book, but especially so for a consumer-oriented list.

Download the full study here.

A word of caution though: just because a study says that you should send your emails on Wednesdays, don’t just blindly follow this advice. Find out what the best day and time is for your business, you should run your own tests.