Summer solstice

The summer solstice is a natural phenomenon that occurs in the Summer when the angular distance between the Sun and the equator is at its smallest. The significance of the summer solstice is in the reversal of the gradual shortening of nights and lengthening of days. It is also the longest day of the year.

Only during the winter solstice, the summer solstice and the equinoxes are the Spirit World and the mortal world close enough together to interact freely with each other for a time long enough to accomplish something.

History
In the Summer of 100 ASC, Avatar Aang used the power of the summer solstice to cross over into the Spirit World where Roku was waiting for him. The purpose of this journey was to teach him about Fire Lord Sozin's machinations of world conquest and the eventual dawn of the War. Roku told Aang that he needed to understand how the war began, if he wanted to know how to end it.

Aang was confronted with the solstice again when he and Zuko traveled to the Sun Warriors' ancient city. They couldn't enter the sacred room until the light of the Solstice would reach the sunstone situated above the door. However, Zuko redirected the sunlight with one of his swords, fooling the celestial calendar into opening the door prematurely.

Connections with the Avatar
During the solstice, the boundaries between the Avatar World and the Spirit World are almost nonexistent. This makes it easier for the Avatar to enter the Spirit World, even without him noticing it.

The energy that comes from the sun on a solstice can trigger the Avatar State. Avatar Roku demonstrated this by crossing over into the Avatar State when the sun reached his eyes on the moment of the winter solstice.

Other Types
The Sun Warriors used a celestial locking system that was triggered by the angle of the light on the solstice to protect their sacred hall. Only when the light reached a large orange gem, located in center of the courtyard, could the door be opened.

Behind the scenes

 * Many cultures celebrate various combination of the winter and summer solstices, the equinoxes, and the midpoints between them, leading to various holidays arising around these events.
 * In many cultures the solstices and equinoxes traditionally determine the midpoint of the seasons, which can be seen in the celebrations called midsummer and midwinter. Along this vein, the Japanese celebrate the start of each season with an occurrence known as Setsubun. The cumulative cooling and warming that result from the tilt of the planet become most pronounced after the solstices.
 * The traditional East Asian calendars divide a year into twenty-four solar terms (節氣). Xiàzhì (pīnyīn) or Geshi (rōmaji) (Chinese and Japanese: 夏至; Korean: 하지(Haji); Vietnamese: Hạ chí; literally: "summer's extreme") is the 10th solar term, and marks the summer solstice.

Escape from the Spirit World

 * Escape from the Spirit World: Avatar Roku Online Comic Book