Knowing when to send emails to best effect is an eternal question, and the decision is usually based on a gut feeling. However there is a way to make this a little more scientific using Google analytics.
This post on the e-consultancy website shows you how to analyse conversions by time of day, and by days of the week. Once you have this information, you can make an informed decision as to the best time to send your emails.
Using this type of analysis has caused us to re-think the scheduling of a number of our email campaigns