An email broadcast can be a sales pitch or regular newsletter or anything you are sending out to your list.
There are several studies out there, and the actual best answer is to figure it per email address you are sending it to…because it is different for each person on your list. But that is very expensive and difficult to pull off for a lot of software companies that provide email marketing services.
So the next best thing to do is to use a service that takes sort of an average of the best time to send the email broadcast and use it. For example, MailChimp (I swear I am not affiliated with them, I just write about them a ton because they are a great service!) lets you send at the most optimal time for your list once you choose the day.
If your particular brand of email marketing service doesn’t permit sending your broadcasts out by analyzing the best time for your list, then I suggest a little split testing. Send each newsletter at various times and see what the results are and go with that, remember to retest periodically.
And finally, if you are looking at this and thinking “pfssssssh I am not gonna do all this work to improve my open rate,” I totally get it! Then your best bet is to land in the inboxes first thing in the morning, first thing after lunch or even post-dinner time in the evening.