June 21 is the first day of Cancer season, and it is often the first official day of summer. It is also often the date of the summer solstice, the longest day of the year. People born on June 21 share the zodiac sign of Cancer. Cancer is the fourth sign of the zodiac, which the sun enters at the summer solstice in the Northern Hemisphere). Those who are born from approximately June 22 to July 22 (depending upon the year) are born under Cancer). Therefore, June 21 is the last day of the Gemini zodiac sign and the first day of the Cancer zodiac sign.