God Vishnu incarnated as NarSinha in order to keep up the honour and devotional respect of devotee Prahalad.
There is a mention of NarSinha avtar in the Ramayana, the Mahabharata and in latter works of all philosophical schools.
The fast on this day is undertaken by all men and women. After bath and purification, Vedic hymns are enchanted, NarSinha image is worshipped after donation and alms.
At sunset aarti is done in the temple. The blissful
award of this fast is freedom from earthly difficulties.
Kashyap had two wives Aditi and Diti. Aditi was the mother of gods and Diti, the mother of two sons, Hirnyakasha and Hirneykasipu. The former was a cruel ruler, whom Varaha incarnation killed, the latter in order to take revenge underwent severe austerities and obtained a boon from B'rhma (details under Holi festival), by virtue of which he could not be killed by any man or animal in day or night inside or outside, by arms or by scriptures.
Hirneykasipu did not want his son Prahalad to undertake devotion and Bhakti to God, but Prahalad did not abstain. On the fourth of Baisakh Shuki, when Prahalad was lost in his meditation; Hirneyksipu pulled his sword and jumped on Prahalad saying, "You as my son adore my enemy. I will kill you now, call your god to save you." Prahalad said, "My respected father, God is omnipresent in you and in me; in your sword and in this lifeless pillar." Outraged Hirneykasipu struck his sword on the pillar, which tumbled down and under a thunderous uproar, Nar Sinha avtar appeared, who had half body of a man and half of a lion, picked up Hirneykasipu on his knees, put him on the threshold, tore open his stomach with nails, and put an end to Hirneykasipus' life.
Prahalad's devotion is quoted, honoured at every occasion (Holi festival in particular).
This legend gives a moral lesson that when hatred develops in the mind of an individual towards pious souls, he will perish before long and that when the demon seeks to harm his bright souled son (Prahalad), he (Hirneykasipu) will be slam.
Prahalad was a gifted child, who because of divine grace, had no carving for material objects, since he looked upon them as unreal. He had thoroughly conquered his desires, had relinquished the characteristics of parental demon and had become a votary of the lord.