I think  I can assert  that the formula to get what is the perfect day to send a newsletter does not exist .

Although there are studies that say that the best day is Wednesday,  I say that "depends". Marketing by email is not  "2+2 = 4" because our target is a specific person, belonging to a particular sector and with particular opinions.

The best is adapt the contents and frequency of our newsletter to those who will receive it. One of our clients sends a newsletter where you can find suggestions for weekend and sends it successfully every Friday. Who call those who have developed the study and tell it  them?

My experience tells me that what we should do to get success with open rates of our newsletters is test, test and test. Select  one day and send the newsletter analyzing results. Select another day and analyze.... Try  and try and see what happens ...

It is laborious, but very effective. It's worth it!