Winter solstice

This is a natural phenomenon that occurs when the sun is at its greatest distance from the equator. It is also the shortest day of the year. In the Avatar world, as the solstice approaches, the natural world and the spirit world grow closer together until the line between the two is blurred. During the solstices and equinoxes, the spirits can exert incredible power in the natural world. This is also the ideal time for people (with the ability) to enter the spirit world.