The best time to send marketing emails is around noon or 4PM (assuming the recipient is in your timezone). This is likely because people will be opening their emails during their lunch break, or near the end of their work day when their interest is starting to wane.
This article by Optinmonster summarizes several studies done by various email marketing businesses. Trying to plan for when your customers have the most free time is generally a good idea – however, as is the nature of emails, even if you send one at 3AM, it’ll still be in the customer’s inbox, ready for them when they wake up.