Recall February 24, 1998. The Nagano Winter Olympics had just ended, Titanic was the No. 1 movie, Destiny's Child's "No, No, No" was climbing the charts.
It's also the day the Buffy the Vampire Slayer episode "Passion" aired. Remember? That's the one where Angel, in his evil, soulless form Angelus, kills Ms. Calendar. At the time, it came as a complete shock; even now, it's still enough to get those tear ducts going.
Fifteen years after it first aired, "Passion" does still stand out as a cataclysmic episode, but more important for the development of the show, it stands out as a distillation of what BTVS did exactly right: Yeah, Buffy and her pals are dealing with incredible forces of evil, but what they're really doing is dealing with being teenagers. Oh, Angel murders someone, but telling Joyce about the night he and Buffy "made love" is almost as egregious; certainly we have all prayed for death when forced to talk to our mothers about sex.
Ms. Calendar's death broke fans' hearts, it broke Giles's heart, it broke Willow's heart, it hardened Buffy's resolve — and it cracked the show wide open, ushering in its darkest episodes and pushing the teen genre to include a new level of surprise and sadness.