Gemini, or the "Twins" (Castor and Pollux), is one of the sky's most visible constellations in the Northern Hemisphere, and one of the twelve Signs of the Zodiac. Its name is from the Latin gemini "twins", the plural of the adjective geminus "doubled, a double, twin, something two-fold". It is the third astrological sign, falling between Taurus and Cancer. As a sun sign, it represents the dates of May 22 through June 21.
In Greek and Roman mythology, Castor and Pollux were half-twin brothers known together as the Dioscuri. Their mother was Leda, but they had different fathers; Castor was the mortal son of Tyndareus, the king of Sparta, while Pollux was the divine immortal son of Zeus, who seduced Leda in the guise of a swan. Though accounts of their birth are varied, they are sometimes said to have been born from an egg, along with their twin sisters Helen of Troy and Clytemnestra. The brothers were known as outstanding horsemen and famous for their loyalty to each other without rivalry or jealousy between them. They were inseparable. When Castor was killed, his immortal brother Pollux, inconsolable, petitioned Zeus to bring him to Olympus.
The Zodiac: Myths and Legends of the Zodiac Signs (astrologyclub.org)