I wonder: everyone in academia handles a lot of email, on all aspects of their work. I mean, emails about conferences, emails about journals and papers, emails about research, teaching, supervized students, and so on. So if you have a big announcement to send to many people (eg seminar or conference) does it matter WHEN you send it? I mean, is it better to send it on Wednesday afternoon (middle of the week) rather than Saturday evening (middle of week end)? I imagine if people receive the email at time they are busy they possibly will overlook it.
First, I don't think it makes much difference. Researchers deal with information, and a lot of that information is communicated through emails nowadays. So, most researchers I know are very careful about their emails and read them thoroughly. Especially if you write to people who know you, your name should be enough for them to read your email through anyway.
But, I can understand if you have an important announcement to make to a large list of people (who don't all know you personally), you may want to “micro-optimize” this. I have done it in the past: having a call-for-papers email ready on Saturday, and waiting ’til Monday afternoon (US time) to send it, thinking people who read it with a fresh mind, and not alongside the batch of “week-end email” that they might triage on Monday morning. (Some people stay connected during week-ends, of course, and for those it makes no matter.)
Marketing experts advise to send emails on Tuesday, Wednesday and Thursday. They also advise to send the email early in the morning, so that they will be at the top of the pile for those that check their emails right after waking up/arriving at the office.
Sunday afternoon is also a good choice. If the receiver reads emails during the weekend, you will be one in a very few, and if the mail is read on Monday, it will be at the top of the pile.
My short answer is that I only send important email within working hours avoid early morning late afternoon hours. I usually get a very high reply rate with this technique as opposed to send email at midnight.