Cancer symbol is a crab and falls from June 21st to July 22nd. According to mythology In the myth, Hera sends the crab to distract Hercules while the hero is fighting the Lernaean Hydra, the serpent-like beast with many heads and poisonous breath, represented by Hydra constellation. When the crab tries to kill Hercules, the story goes, Hercules kicks it all the way to the stars.
In another version, the crab gets crushed instead and Hera, a sworn enemy of Hercules, places it in the sky for its efforts.
However, she places the crab in a region of the sky that has no bright stars, because despite its efforts, the crab was not successful in accomplishing the task. Cancer does not have any stars brighter than fourth magnitude.
Info from costellation-guide.com
picture from astrologyclub.org