A typical year at Hogwarts:
months of school: nothing noteworthy at all. Harry and friends go to classes, break a few rules and meet a couple of quirky new people.
Last 4 weeks of school: there's something evil in Hogwarts! Good thing Harry knows exactly how to fight it!
Last day of school: climactic showdown between Harry and said evil thing.
(And they all lived happily ever after.)
Why do the villains choose to reveal themselves only when the school year is nearly over? Is Voldemort following some cosmic calendar that dictates his evil days and his sit-around-and-not-do-anything-very-evil-at-all days?