Narsimha Dwadashi refers to the day when we worship Narsimha Avatar of Lord Vishnu. The festival falls on a Dwadashi, the twelfth day of the fortnight during Krishna Paksha in the month of Phalgun. While one form of Lord Vishnu is worshiped on each Dwadashi, Lord Narsimha, the man-lion avatar is offered prayers to on this day.
It is said that just as Lord Vishnu had saved his devotee Prahlad from the clutches of Hiranyakashipu, similarly he saves all his devotees from problems in life. He makes us confident, fearless and empowered, thus helping fight off all the problems. A fast observed on this day helps wash away the sins of the past life. It was the story of Prahlad in which Lord Vishnu took the form of Narsimha.
Jaya and Vijaya were the two gatekeepers of Lord Vishnu. Once, they got a curse from four boys that they would have to take birth as demons on the earth and then would be killed by Lord Vishnu himself. As a result, these two gatekeepers were born as demons named as Hiranyakashipu and Kiranyaksha. The younger of the two, Hiranyaksha was a tyrant ruler who got killed at the hands of Lord Vishnu in his Varaha avatar. This set the elder brother Hiranyakashipu really upset. Hiranyakashipu now wanted to take revenge of his brother's death.
With this in his mind, he started a penance in order to please Lord Brahma. A pleased Lord Brahma appeared before him and asked for his wish. A clever Hiranyakashipu, asked a boon that he should not be killed by God, human, an animal or by a demon, neither inside nor outside the house, neither on earth, nor on the sky, neither by a weapon made of wood, nor of metal and nor stone, neither should he be killed in the daylight, nor at night. As per his request, Lord Brahma granted him the boon and disappeared.
It is said that after getting the boon, the demon started misusing his powers and causing atrocities everywhere around. Not just this, in order to take a revenge of his brother's death, he started torturing the devotees of Lord Vishnu. He wanted people should worship him and not Lord Vishnu. However, his own son, named Prahlad, had started worshiping the Supreme lord Vishnu by then.
Initially, Hiranyakashipu tried various ways to change the mind of his son, but all his efforts went in vain. As a last option, he sought the help of his sister named Holika. Holika had a magic cloak, which could protect her from fire. She had obtained it as a boon. The idea now was that she would sit on fire with Prahlad in her lap. While the cloak would protect her, the kid would get burnt.
The day came when they were to execute her plan. However, much to their surprise, the Magic cloak caught fire and the lady got burnt and the kid got no bruises at all. This day is remembered till today as Holika Dahan (meaning burning of Holika).
However, the demon did not stop there. There goes and incident when he asked Prahlad where his God Vishnu stays. To this Prahlad said that God was everywhere. An amused Hiranyakashipu kicked the pillar he was standing near asking if God was there in that pillar also. And to everybody's surprise, the pillar collapsed with a thunder.
And with a roar, a half man-half lion creature came out of it. It was none other than Lord Vishnu in his Narsimha avatar. Neither a complete human, nor God, demon or animal, he dragged Hirayakashipu on his lap (not on earth or sky, but God's lap) and killed him with the long nails of his paws. It was the time of sunset, neither day nor night. Thus, died the demon Hiranyakashipu and Prahlad was made the new king.