During the first few weeks of classes (and indeed throughout the year), a senior will often announce at any given house's dinner that "Ditch Day is Tomorrow, frosh!"
Most of the freshmen have heard about ditch day, but know very little about it. (I for one hadn't the slightest clue what a stack was until well into first term my freshman year.) So they tend to be rather confused about all this business about it being tomorrow. The real story is that some time ago, the administration started demanding to know when ditch day was, since it has a rather broad effect on campus activities (all classes are actually cancelled, for one thing). In response, the use of Tomorrow with a capital T as a synonym for ditch day was spawned as a sort of protest. (ADMINISTRATION: "Hey seniors, WHEN IS DITCH DAY?" SENIORS: "Ditch Day is Tomorrow!" You get the idea.)
Nowadays, the school actually is informed in advance, so "Ditch Day is Tomorrow!" is used primarily as a tool for taunting frosh into teary-eyed submission.