In ancient Hellas, Cancer was called Karkinos (Καρκινος), who was an individual crab or crustacean. Hēraklēs, son of Zeus and the mortal Alkmene (Ἀλκμήνη)--who was a bane in Hera's life, simply for being born--was stricken mad by the Queen of the Gods and killed his five sons by his wife Megara (Μεγάρα), oldest daughter of Kreōn (Κρέων) of Thebes. When he was released from his madness by a hellebore potion--provided by Antikyreus--and realized what he had done, he cried out in anguish, and went on a long journey to cleanse himself of the miasma caused by these killings.
First, he visited the oracle at Delphi, who, unbeknownst to him, was whispered to by Hera. The Oracle told Hēraklēs to serve the king of Tiryns (Τίρυνς), Eurystheus (Εὐρυσθεύς), for ten years and do everything Eurystheus told him to do. Eurystheus gladly provided Hēraklēs with these labors--ten of them, one for each year--and eventually ended up adding two more, resulting in the Twelve Labors of Hēraklēs. Hēraklēs was told to: slay the Nemean Lion, slay the nine-headed Lernaean Hydra, capture the Golden Hind of Artemis, capture the Erymanthian Boar, clean the Augean stables in a single day, slay the Stymphalian Birds, capture the Cretan Bull, steal the Mares of Diomedes, obtain the girdle of Hippolyta, Queen of the Amazons, obtain the cattle of the monster Geryon, steal the apples of the Hesperides, and to capture and bring back Cerberus.
On his second labor, slaying the Lernaean Hydra, Hera send Karkinos to distract Hēraklēs while he fought. Unfortunately for the animal, Hēraklēs wasn't very impressed. He either kicked the animal so hard, that Karkinos flew so far into the sky, he can hardly be seen, or he squashed the crab, and Hera placed him into the sky for trying. She did, however, dim his stars because he had failed at his task.
The constellation Cancer is visible at latitudes between +90° and −60°, and best visible at 21:00 (9 p.m.) during the month of March.