What is the "Sudden Departure" in the novel, The Leftovers?

The Sudden Departure occurred three years before the start of the story. It was the day on October 14th that certain people in Mapleton disappeared into thin air. They just evaporated according to some Eyewitnesses. Some called it the Rapture; others refused to attach any religious meaning to it. What everyone had to agree on was that October 14th devastated the community. Sudden Departures also occurred worldwide on the same day.