A solstice is defined as the point during the Earth's orbit around the sun at which the sun is at its greatest distance from the celestial equator, while an equinox is at its least distance. A solstice is characterized by either an extremely long night or an extremely short day.
Meanwhile, during an equinox, the day and night cycles of the Earth are nearly equal. Of course, due to planetary wobble and stellar shift, there is no point when both day and night cycles are exactly equal, but they do come close. There are two equinoxes and two solstices each year. Equinoxes occur on approximately March 21 and Sept. 22, and are referred to as the vernal and autumnal equinox, respectively. Solstices occur on approximately June 21 and Dec. 21, referred to as the summer and winter solstice, respectively. However, due to the 23.5-degree tilt of the Earth's axis, these seasons are reversed based on which side of the planet's equator a person is located on. This is because from December to June the Northern Hemisphere is turning toward the sun while from June to December the Southern Hemisphere is turning toward the sun. However, the naming conventions were determined by astronomers in the Northern Hemisphere, hence their relation to the seasonal cycles found in that portion of the world.