This is a tricky question because the answer depends on your list, your industry and a plethora of factors. Here are a few tips that can help you decide the best moment for you to send it based on your situation.

Peak periods

1) More emails are sent during the week than on weekends, with an increase in midweek.

2) More emails are opened during the day.

But peak periods do not mean better results. A full mailbox after the holidays may lead to a cruel sort or worst, massive unsubscriptions.

Tip: try to divide your list into a few chunks and test different days/hours to dispatch your mailing to see what works best for you.

Personal or business?

My members are mainly business customers

If your emails are to be received at the office, send them early in the morning on weekdays. Another interesting period: the return of the dinner. To avoid:  late afternoons and Thursdays and Fridays.

My members will receive my emails in their personal mailbox

The best moments to write are then in the evenings or on weekends.

Conclusion: there is no best timing

Even if you read all the advices available on the web or in the world, it is impossible to find an absolute and verifiable answer to this question. Know that if your email has a real value for your members, it will be well received regardless of the time of your mailing.