Summer solstice

The Summer Solstice is an event that occurs in the Summer. It is a natural phenomenon, occurring when the sun is at the closest angular distance to the equator. It is also the longest day of the year. Both, the Winter solstice and the Summer solstice, and the equinoxes are the only times that the spirit world and the mortal world come close together enough to interact freely with each other for enough time.

History
The first known and documented siting of the solstice was during 54 BSC. Avatar Roku was staying at the Fire Sage temple, attempting to master the massive power of the Avatar State. Roku began to become very impatient meditating and such, so decided to use the near Summer Solstice to speed it up. It reached the gem at the top of the entrance, and he automatically triggered the state. He could not control it, however, and so Fire Sage Kaja was forced to save him from it.

Hundreds of years later, in the Summer of 100 ASC, Avatar Aang used the power of the solstice to enter the Spirit World to go on a journey with Roku. This story was to teach him about Fire Lord Sozin's factions of the War. A few months later, he and banished Prince Zuko went to the Sun Warrior's Ancient City, where they are given the knowledge to wait to enter a temple until the day of the Solstice, which would be the next year. Impatiently, Zuko faked the glow and entered the temple.

Connections with the Avatar
During the Solstice, like the Winter Solstice, the boundaries between the normal world and the spiritual world is very thin, almost non-existing. The Avatar, therefore, is easily allowed to enter the Spirit World and other spiritual connections to their past lives.

Due to the now strong connections with past lives, Avatars can as well enter a much more powerful and sparingly uncontrollable Avatar State. Roku did so, unleashing such a powerful wake of it, he semi destroyed the entire Fire Sage temple.

Other Types
The Sun Warriors, an ancient and viewed as extinct followers of dragons, use the Solstice to detonate their sacred hall that demonstrated the way to do the Dancing Dragon. The Solstice is not limited to just this, as Prince Zuko could ignite it by using a direct gleam from sunshine in the celestial calendar.

Appearances and References

 * Escape from the Spirit World (Appears in flashback)
 * The Avatar and the Fire Lord
 * The Firebending Masters