Summer Solstice can occur on June 20, 21, or 22, depending on calendar events such as leap year and when the Solstice begins relative to Coordinated Universal Time.
According to NASA, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.
This longest day of the year marks significant seasonal changes for the United States. Most importantly for coastal states, the 2013 Summer Solstice marks the highest exposure of the northern Atlantic and Pacific Oceans to the Sun’s heating radiation. The rise in ocean temperatures then provides the energy necessary to produce stronger hurricanes later in the Summer.