Moon

The Moon appears during night time.

Grass blocks or Saplings do not grow in moonlight unless there is a Torch (regular or Redstone) or other light source nearby.

Mobs
Hostile Mobs will spawn around dark areas of a normally generated map. When the moon is up, hostile skeleton, creeper, slime, zombie and spider mobs will appear and attack the player. When the sun comes back up Skeletons and Zombies will burn in the sunlight and drop items on death. Spiders will still be there but are peaceful mobs during the day (unless aggravated). Creepers and slimes do not go away.(See also: Difficulty level.)

Trivia

 * Night time lasts 7 minutes, plus 1.5 minutes each of sunrising and sunsetting.
 * You can't see the moon if you have the small or tiny render distances.
 * The moon is 8 x 8 blocks.