What celestial event occurs every 10,000 years in the Avatar world?

The celestial event that occurs every 10,000 years in the Avatar world is Harmonic Convergence. It is a rare alignment of the planets that dramatically strengthens the connection between the physical and spirit worlds.

In The Legend of Korra, the event occurs during Book 2: Spirits. Korra learns that Harmonic Convergence will allow the dark spirit Vaatu to escape his prison if the southern and northern spirit portals are opened. The event also gives energy to the portals and enables major changes in the world's spiritual balance.

Harmonic Convergence has consequences beyond the immediate conflict. During the event, the boundaries between worlds become unusually thin, and the energy released helps create new spiritual possibilities. One of the most important results is the return of airbending to people around the world after the portals remain open. Sozin's Comet is another major celestial event in Avatar history, but it returns every 100 years and amplifies firebending rather than occurring every 10,000 years.