In Lost, Oceanic Flight 815 crashed on the island in 2004.
The crash occurs on September 22, 2004, after the aircraft breaks apart in flight. The date is central to the series because the survivors initially believe they are stranded in an ordinary, remote location.
Lost later reveals that the island has unusual properties involving time, memory, and causality. These developments make the year 2004 an important anchor for the main storyline, even when episodes show flashbacks, flash-forwards, or time travel.
A common mix-up is confusing the crash year with the year the series premiered. Lost premiered in 2004, and its fictional crash also takes place in 2004, but the events are separate parts of the story.