I wish there were some more universal dates, like those set by the Council of Graduate Schools and Council on Graduate Medical Education for students. No one entering grad school has to make a decision before April 15. All the recruitment is done mostly over the preceding two months. Oh how that would make job searcing so much easier.
Ah yes -- but as a former DGS, I can tell you that people say "yes" at 9 a.m. on April 15th, call back to say "no" at 11 a.m., next person on the list (well actually 16th person, since there have already been turn-downs) is actually reached at 12:30 and says they'll call back, which they do at 3:00 to say "no," by which time . . .
With incoming grad students, all there is to negotiate is: fellowship or TAship? TAship or RA position? How big is the stipend? What benefits are covered? In most cases, the negotiations with an individual faculty member take at least a week to discuss all the issues, (space, equipment, teaching schedule/load, research money, salary, early leave, money for conferences, reduced load, start-up funds, etc.) even if there isn't anything complicated such as spouse line involved. And while grad directors know what can be done -- it's usually set in stone -- department chairs have to go to the dean, sometimes to facilities management, often the provost, sometimes have a talk with the union or the legal officer - - - and at a university such as mine, which is big enough that across campus and professional schools, 80 or 90 TT faculty are being hired every year . . . . can you imagine doing all of this within even the last two weeks of April? Wouldn't work!