June 21 - July 22
As with many stories behind the astrological signs, Cancer the crab's starts with Hercules.
Hercules, the strongest mortal in Ancient Greek mythology and the son of Zeus, the god of lightning and all the other gods, was born by a human woman. Hera, Zeus' wife and the goddess of women/marriage, was offended by the baby Hercules and attempted to kill him many times.
One of those attempts brings us to Cancer. One of the 12 tasks that Hercules had to complete was to defeat the Hydra, a nine-headed sea monster. When it seemed like the Hydra would be defeated, Hera sent a giant crab to aid it and defeat Hercules at last. During the fight, Hercules' crushed the crab's shell and killed it and then went on to defeat the Hydra.
As a reward to the crab for his services, Hera placed his shape in the night sky. He is surrounded by dim stars and is dim himself to signify that he did not succeed in his task.