Epochal describes events so important and significant they have the power to usher in a new epoch. In other words they're "epoch-making."
The big bang. The moment when man learned to make fire. The day your braces were removed. These events are epochal in that they mark the dawn of a new era, or epoch, of time. Both epoch and epochal derive from the Latin epocha, which in turn derives from the Greek epekhein, which combines epi "on" with ekhein "to hold," and means to pause or take up a position. Think of epochal events as worthy of an arrow on the timeline of history.