At the Gathering, the three Clans meet at the plateau under a truce. There, they share tongues and news of the happenings between the Clans. However, this truce can be broken and when it is StarClan is said to send clouds over the moon to signal this.
For roleplay purposes the Gathering times are located here. Please refer to that page for when the next meeting is held. They are a week long to allow all users to participate if unavailable.