Ullambana 

Festival of the pretas or ‘hungry ghosts’, traditionally celebrated on the fifteenth day of the seventh month when offerings of food, money, and clothing are made to departed spirits. The ritual derives from the Ullambana Sūtra and the practice is said to have originated with offerings made by the Arhat Mahāmaudgalyāyana when he realized through his magical powers that his mother had been reborn as a hungry ghost. The festival has been particularly important in Chinese Buddhism from the T'ang dynasty, and was first celebrated in 538 ce.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)