History/Practices:  Blades of grass from Kusa are sacred to Ketu.

Mythology:  Birth Myth:  During the giving out of the ambrosia to the gods, one of the Asuras (demons) disguised himself as a god and sat among them.  Just as he took a mouthful of ambrosia, Surya and Chandra realized the fraud, and Vishnu quickly cut him in two, thereby giving us Rahu and Ketu.  However the power of the ambrosia caused the two pieces to continue to be alive, and so Brahma changed them into planets and sent them to the heavens.  Rahu continues to be the enemy of Surya and Chandra. Eclipses are when Rahu tries to devour them.

Sources:

Moor, Edward.  The Hindu Pantheon.  Los Angeles: Philosophical research society, 1976.

Thomas, P.  Epics, Myths and Legends of India.  Bombay, India: D. B. Taraporevala Sons & Co. Private Ltd, 1961.