At midnight, Growtopia /time, the comet will fly pass Growtopia for 24 hours. This will happen only once per month, ALWAYS occurring on the 28th. The first event happened on the 28th of November 2014 when update was introduced.
The message players got before the first event took place said: Some astronauts are working on something.
If you are online at midnight Growtopia /time on the 28th, you will see this message instead: What's that in the sky?? A Comet is blazing a trail through the night! It will only be here for 24 hours...
It changes all world's weathers to dark green hills which appear to be at night with a comet in the background moving accordingly to the time. Comet Dust and Antimatter Dust will spawn during that time in worlds depending on how many people there are (they don't fall from the sky, they just spawn like fireworks)
Some new clothes could be found such as Cosmic Skirt or Velour Space Uniform. These items can be spliced with the Starseed or Galactic Starseed (which you can get by tapping 200 comet dusts or 200 antimatter dusts on yourself) . Before the first night, players could obtain the clothes from rare clothes packs but was later removed.
During the Night of the Comet, the Geiger Counter will be really hard but still able to used. It would always detect radiation where ever you go, since the comet emits radiation. If you manage to find something during the Night of the Comet, you will get the achievement "Atomic Luck".