When Season 4 launched, the comet slammed into the island, although it came in multiple fragments which impacted different regions around the map. Small comet fragments began falling down around the island at the end of Season 3.
Telescopes and various signs were placed at high-altitude areas around the map, as the inhabitants prepared for its arrival. As it grew bigger and brighter over time in the sky, people realized it was a comet, heading straight towards the island.
In Season 3, players saw a bright blue object in the sky.