Until now, the consensus was that the fateful explosion of Mount Vesuvius happened on August 24, 79 CE — this notwithstanding the presence amid Pompeii’s ruins of warm-weather clothing and the remains of autumn fruit. Now excavators have uncovered graffiti with the date October 17. Archaeologist Kristina Killgrove explains why it’s almost certain that this graffiti is from just before the eruption and not a prior year, and why the particular date of the catastrophe matters.