Generally speaking, when you tap a tree it is analogous to you getting a cut on your body. Just like your body creates a scab to close off the wound and prevent future infection, the tree will seal-off the taphole. Once you drill that hole, the clock starts ticking. In a typical bucket or gravity tubing scenario you have about 6 weeks of usable taphole life. If you tap too early and then an extended cold spell hits, you run the risk of missing some good later season runs. This is why folks stress on tap timing. And to be honest, you don't know what the "correct date" is until after the season is done!
One interesting read (there are others): http://www.uvm.edu/~pmrc/tapping.pdf