Sheetla, the Goddess presiding over small donkey is worshipped on Chaitra Krishna Ashtami (eighth day in the dark fortnight of the Hindu lunar month Chaitra). It is believed that worshiping the Goddess Sheetala on this day protects against small pox etc. Goddess Sheetala is known as the Goddess of disease epidemics and disaster. Fresh food is not cooked on this day; food is cooked on the previous day. After worshipping Sheetla Mata on Chaitra Krishna Ashtami (eighth day in the dark fortnight of the Hindu lunar month Chaitra) stale food is consumed.
Story Once there were two families one was of a King and other of a servant. Sons of these two families got infected with small pox. The servant, who was a God-fearing virtuous person, used to worship Sheetala Mata and took the suggested precautions in the name of the Sheetla Mata (isolation, no fried cooking and eating of hot food). Condition of his son recovered day by day. In the other case, the King who arranged big Yagya (fire sacrifice), donated a lot, allowed the son to eat things of his choice. The condition of King’s son deteriorated day by day; at last the King consulted the servant and was advised to observe proper precautions, fasting and worshiping of Sheetla Mata. The King did as per the advice, his son recovered fast.
Aum Namah Shivaye
Source: Book: Hindu Festivals By Shashi Kant Kaushik