From the information we got from the testers to define the best hour to send the email, the conclusion is that it is 9:00. But we didn't reached a conclusion about the best day.
@tiagobugarin proposed we try to improve this answer with data. We need to align a strategy to do that.
Since we need to learn and design a strategy, and this can take some effort, I'm not including this issue in the backlog yet.