A Gathering is a meeting of the Clans, held at the night the full moon (thus, once each moon).
On Gatherings, Clan leaders announce current affairs and significant events in their Clan (like appointing new apprentices and warriors, or death of a cat). They also use this time, if necessary, to ask for help from others, such as when WindClan asked RiverClan for drinking rights in their territory, when a drought shortened their water supply.
Warriors use this chance to meet their friends from other Clans and exchange information. Many cats will use a Gathering to try to determine the strength of another Clan.
The Gathering is held on the night of the full moon, because it is the only time that the moon is reliably bright enough to see well. The leader, the deputy, several warriors, apprentices, and elders participate from each Clan. If a leader cannot attend, the deputy will speak on behalf of that Clan.
Once arrived at the Gathering territory, the leaders sit on a high spot (for example, a rock or a tree branch) where they can be seen and heard well while talking to the warriors and to each other. Deputies sit close by. It is considered disrespectful if any other cat speaks from atop the leader's spot, unless specifically asked.
As decreed by SpiritClan, the nights of the Gatherings are a time of peace, and if there is any violence, StarClan sends a cloud to cover the moon signifying their displeasure; usually the Gathering ends if this happens.
The Gatherings are held on Free Mountain.