To the best of my knowledge, the summer solstice, or perhaps the estival solstice, which is also known as the midsummer, occurs when one of the earth’s poles is tilted towards the sun. It happens about twice in a year and once in each of the hemispheres. From an article I once read, the summer solstice occurs when the sun is at its highest or perhaps the day with the longest period of daylight.
Within either the northern or the southern hemisphere, there’s steady and continuous daylight around the summer solstice, as such, the axial earth tilt towards the sun at about 23.44 degrees; in the same light, the sun’s declination from the celestial equator is about 23.44 degrees. The summer solstice, in most cases, occurs during summer, most times around June in the northern hemisphere and December in the southern hemisphere, depending on the shift in the year’s calendar.