This is the date that an admissions officer kept quoting me recently, too, without really giving any clue as to its significance. I think she said something along the lines of "if you've not heard anything prior, call back on the 31st and I'll have a clearer idea of where you are in the queue". I didn't take much notice of it at the time, because she then proceeded to log into my application specifically, and amended her advice to "oh, actually, you should definitely hear something in the next week".
That was 2 weeks ago.