For many schools, the 15th is the deadline. More specifically, a candidate may be required to have the deposit postmarked by then. The letter could take several days to travel to the otherside of the country. Then the committee has to meet sometime after all that (usually on a Friday), and send a letter out, which could also take a couple days to reach you. It may take a week or two, depending on whether they decide on the 18th or the 25th, before you hear anything.