When sending out an advertising campaign, timing is everything. Many studies have been done, and they all agree that the ideal days are Tuesday through Thursday.
Monday, being a busy day used for planning, is not ideal. Furthermore, your email would likely be lost in the flood of unread emails from the weekend. As of Tuesday, business clients tend to read their emails, but once noon on Friday arrives, many are already in weekend mode.
Other than the day, you must consider the time of your mailing. An email received between 8 and 10 a.m. will be among the first emails read upon arrival at the office. An email received too close to lunch time or near the end of the day will not have the desired impact.
If your emails are aimed at business clients, concentrate on Tuesdays through Thursdays, but if your clientele is primarily a consumer one, Sunday may be a good option. Many consumers shop online on Sundays. If in doubt, try it out. Send your next email campaign on a Tuesday and the following one on a Sunday. You will then be able to assess the effectiveness of each campaign. It all comes down to your target market.