What’s the best day of the week to do an email campaign?

This is highly dependent on your audience and the type of content you're sending, but also keep in mind, humans are wonderful and unique creatures and our preferences can vary widely, even when we're within the same demographic. The engagement data from your experiments may help you identify what your audience seems to like best. Review your positive engagements (opens, clicks, replies, conversions), as well as ones suggesting negative sentiment (unsubscribes, spam complaints, angry rants via reply). Make hypotheses and then keep testing...the data should make it clear . If not, it's possible there really is no best day of the week for your mail...your audience just likes it whenever you send it. :)

Depends on your target audience. its best to test and optimise along the way

Hi Larry, this is a great question but it really depends on the target audience and the industry you are promoting. I recall making studies years ago in the iGaming industry and we came to the conclusion that Thursday afternoon was giving us the best results in Spain and Spanish speaking LATAM countries. The best way is to make sample test then analyze various KPIs like open rate, clicks and then conversions, it is for sure time consuming but I think its the best way to get your answer ;-) BTW sorry if I repeat what others might have already stated. Best of luck and you know where to reach me for more details :-D Cheers!