18th
Email marketing is tricky. If you have customers email and you do not send out email blasts it can be a huge waste of a very valuable asset, send out emails too much and you will piss your customers off or have them remove themselves from your list.
I have been buying some stuff from Crutchfield which if you are looking for audio/video stuff is really a great company. However they send me a marketing email at least once a week, it is ridiculous. So I have now unsubscribed.
It would have been much better for them to send me a list of big specials at a maximum of once every 2 months. Then I probably would not unsubscribe from them and might have even bought more then I would have otherwise.
Josh Kopelman did a great post that touches on this in May, you can click here to see it.