It is the day in which the hours of light are equal to those of the night.
In the ancient Chinese tradition, it is the moment of transition from Yin characterized by rest, introspection and the depth of winter and Yang with its rebirth, excitement and expansion towards the new.
It is associated with darkness because the types of processes that naturally gravitate toward darkness tend to draw us deeper into ourselves through sleep, meditation, and contemplation.
With darkness, we tend to venture deeper into the Yin realm, our feminine side, which nurtures and welcomes, regenerates and stimulates subtle perceptions.
Yang is associated with "light" because it pulls us into the outside world.
Yang in fact stimulates our masculine side, giving us expression, action and stimulating physical activity.