Last year it was painful. I do remember it was exactly March 13...but that day came after about 2 straight weeks of "we'll let you know on X day," and then no news would come that day, leading to a stream of SDN freak-outs, someone calling, and getting a new predicted date. So I would assume mid-march, but even once they tell you a day, don't cling to it until people start getting emails.
Which, btw, was another brutal thing about last year...they decided to let all rejections/waitlists know by email, but all acceptances came via snail mail. While I love the idea of opening a big, shiny package from Duke, it was brutal sitting around for 2 weeks being terrified of your email...every time I got the "Gmail-Inbox (1)" message, my heart would sink...and then it'd be a false alarm. Some people had an even worse time, because when the rejections started coming out, most came in the same day...but a few people's servers delayed the email over the weekend, so those poor people were sitting there all weekend going, "omg, I didn't get an email...did I get accepted to Duke???" And then come Monday there was that ugly little email...It was very sad 🙁. I reeeaaally hope they don't do it that way again this year....