I've had the best returns on Monday/Tuesday/Wednesday. I always sell more books at the beginning of the week so I decided to test that out with a free promo on the same days and it really worked like a charm. I'm not sure if results would vary between fiction and non fiction, but that's my experience.
Thursday is my best sales day. I run promos on Weds. Thurs. & Fri. But I have to admit, that's not a scientific answer. It's just that I did a beginning of the week with another pen name and it bombed.
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