Deciding on the best time of day or day of the week to send your email newsletter is always tricky. I advise clients to test it, as it depends on so many factors that it's hard to say what works for one will work for another.
One of my clients has found that sending on a Saturday morning works well - but then she is selling lovely home accessories and decorative items to a consumer audience, and maybe the weekend is when they are doing a spot of relaxed browsing and shopping.
B2B clients tend to avoid Monday mornings when inboxes are crowded and people are busy.
I thought this was an interesting way of researching the best time to send - by setting up a keyword Twitter search RSS feed: find out when people are talking about the stuff you're selling, and adjust send times accordingly.
My only comment would be that ideally you would want to survey only your customers/prospective customers, not the whole Twitter population, plus you'd have to allow for some 'false positives' as with any kind of keyword (as opposed to semantic) search. Worth trying though, it costs nothing, and it's a great idea!
