We're often asked when the best time to send mailings might be, and if there are days or times to avoid. While there's no magic send time, there are a few commonly held notions about the timing of mailings. Remember that your best chance of getting people to read and respond is during the first few hours after delivery, so making sure your emails arrive when people are most likely to be in front of their computers (and paying attention) is the real key.
With that in mind, mailings that go out Tuesdays through Thursdays have traditionally fared a little better than mailings that go out on Mondays or Fridays. On Mondays, people are arriving to overflowing inboxes and a full list of to-do's and 10 people saying, "Dude, my weekend was too short." On Friday, it's a simple matter of math, as some of your audience will have headed out early for the lake, or the martini bar, or the martini bar with paintings of lakes on it.
We also try to avoid late night/early morning mailings in general. That's because we know first thing in the morning -- every morning -- people are sifting through a night's worth of emails, and a serious email somewhere in the middle of all that clutter is too likely to get lost in the shuffle.
Other factors, of course, come into play. Like the timeliness of your message, the geography of your audience (are they in one time zone or several?) and the habits of your subscribers (do they check email at work or at home, and how frequently?). Some factors are out of your control, like how quickly servers deliver your emails to their intended recipients -- most of your emails should arrive shortly after you send, but in some cases you may have stragglers that aren't delivered for hours or more (which is why you'll always want to budget several hours for your entire mailing to arrive, and even more time for larger mailings).
So as much as we can all try to find that perfect window of delivery, our advice is to avoid less-than-ideal mailing times when you can, let the timeliness of your message help determine when you send, and use your account's response tracking to see how different days and times affect your response rates. To that end, in addition to telling you who opens your mailings and who visits your website, we also tell you when they opened and clicked. After all, knowledge is power, and we want you to be all-powerful. Insert maniacal laughter here.