Minecraft wiki entry for Zombie_siege, associated text: At midnight each night (tick 18000 in Minecraft time), there is a 10% chance for the game to attempt a zombie siege that night. If a siege is to occur, attempts are made each tick to start the siege until either a siege is successfully started or the sun rises. Specifically, the siege attempt is abandoned when the sky light level reaches 12. However, inclement weather can allow siege start attempts to continue well past dawn since it reduces the sky light level.