Newly appointed warriors are required to stay awake for the entire night after they received their warrior name, as decreed by the Warrior Code. They guard the camp in silence, while the other Clan members have a good rest.
In some cases, this rule cannot be adhered to, and the warrior receives another task. For example, when Squirrelflight becomes a warrior, there is no Clan camp to guard, so she receives the task to explore the lake with other warriors instead of the vigil.
As Fireheart starts his vigil, he notices Tigerclaw's eyes shining back at him. He then reveals silently that he had made an enemy out of the Clan deputy, Redtail as Tigerclaw did the same for him.
Vigil for the dead
If a Clan cat dies, their body is taken to the center of the camp, and several of their Clanmates, usually the cat's family members, close friends, and mentor or apprentice sit vigil for them through the night, sharing tongues for the last time before the elders bury the cat in the morning.
There can also be vigils for cats without bodies and suspected dead, like ThunderClan did when they suspected that Graystripe died or when Ravenpaw supposedly died. There is no burial, but the cats still sit vigil.
Outside of the Clans, some cats have been noted to sit vigils for the deceased out of superstition, believing that those that fall asleep in lieu of sitting vigil will be the next to die. This is told of when Sharpclaw recalls Foxy's brother didn't sit vigil for him, and he apparently died a couple of days later.