This is a frequent question. There are lots of theories but we don't really subscribe to them. First of all, it is difficult to control when the Postal Service delivers your mail so you cannot specify a particular day even if you would like to do so.
Second, we all receive mail every day. Many people feel that Mondays and Fridays are bad days. Monday is the first day back after the weekend and Friday is the day before the weekend, so the theory goes, and therefore people are too busy to have time to read mail.
We believe that a well presented offer sent to an appropriate audience has a good chance of being read regardless of when it is received.
Of course you'd want to avoid mailing before a major holiday and certainly not into a problem area such as New Orleans.
But trying to second guess the best day of the week is generally a waste of time.