Northern hemisphere spring comes to an end and the summer season begins on Tuesday June 21st at 8 UT (3 am CDT) when the Sun ‘reaches’ the celestial coordinates of 23.5o north declination and 6 hours right ascension. With respect to the Earth’s surface the Sun is described as over the Tropic of Cancer, 23.5o, north of the Earth’s equator. At this same time the Sun is just entering the boundaries of the astrological constellation Cancer the Crab. Interestingly about 19 hours later, Wednesday June 22nd at 3 UT (10 pm CDT June 21st) the Sun will actually be entering the region of the Gemini Twins as it crosses the boundary between Gemini and Taurus.
According to the pseudoscience of astrology the Sun, at this date, would be entering the astrological sign of Cancer the Crab.
We know that it is the Earth’s orbital motion around the Sun that causes the sun’s apparent eastward motion among the stars in the background. This is how the Sun ‘reaches’ a celestial coordinate, how it ‘crosses’ the boundaries between constellations, or how it is ‘in’ a constellation.
With respect to the southern hemisphere this is the end of their summer and start of their fall season. So thinking globally my preference has been to use the name of the month to designate the season change. Hence the use of the term June Solstice rather than summer solstice.