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.

Continue reading here.