Solstice.

Solstice is an astronomical event that signifies the beginning of summer or winter. It is the time of year when the sun is most intensely focused on one of the two hemispheres. The solstices are related to esotericism and for many cultures they have a very important symbolism. For example, for some peoples, the summer solstice is a reason for great joy, because it represents the victory of Light over Darkness. The solstice occurs twice a year, on June 21st and December 21st, marking the beginning of the seasons, which are opposite in each hemisphere.