After reading this article my colleague Skip Fidura posted a great piece on time of day testing in email marketing:
The interesting thing is that we have come full circle. In the early days of email it was a free for all but people quickly realised that email was all about getting the right message to the right people at the right time. It was the last part that caused all of the worry.
How would we as marketers know when somebody was likely to be receptive to our message? So, we assumed that all consumers care about is the weekend and applied some "logic" about their behaviour as it related to the weekend and decided Wednesday would be the best day to send.
Some of the more clever email marketers realised that if every body was sending on a Wednesday it would be too hard to achieve cut-through so they overlaid this thought with the previous thinking on the importance of the weekend and said Tuesday was better (although some argued for Thursday).
Throughout the Great Day of the Week Debate, individually we struggled to answer the question, "I have changed to the 'optimal' day and my results have gone down." Luckily as marketers we had a ready made answer to this dilemma. "Maybe our customers are different. We should test it." These individual conversations eventually lead to a groundswell until most everybody was in agreement that we should rigorously test for the optimal day of the week and hey, let's throw in time of day as well.