The most important part of timing your campaigns is frequency.
I’d always recommend more often than once a week, no less frequent than once every 6 weeks. That way you’re not going to be bothering people too much, and neither do they have time to forget they signed up.
As for the magic day or the magic time to send an email. The jury’s out on this. Anyone that claims to have the answer is usually just looking at one set of data. What matters is the habits and preferences of your individual audience.
That said, there’s a few general rules that seem to hold true for most audiences.
Really late in the day is always bad. Times when your readers are likely to be sat in front of their email inboxes are good. So, it’s much better to get in during the morning e-mail check or around lunch than after 5.
Weekends are generally hit and miss, but Sundays provide the best click through rates… possibly because people have more time on their hands. However, the total open rate will be slightly less, as many people will delete an e-mail they see on Monday morning if it’s not something personally addressed to them, or requiring action.
As with much advice about enewsletters, it’s best to get to know your own audience, and be suspicious of people claiming the one best time, or the one best day. Experiment, and track what works.