How Lord saves His Devotee (Bhakt Prahlad)
There was once a demon king by the name of Hiranyakashyap, who won over the kingdom of earth. He was so egoistic that he commanded everybody in his kingdom to worship only him. But to his great disappointment, his son, Prahlad became an ardent devotee of Lord Narayana and refused to worship his father.
Hiranyakashyap tried several ways to kill his son Prahlad but Lord Vishnu saved him every time. Finally, he asked his sister, Sinhika (Holika) to enter a blazing fire with Prahlad in her lap. For, Hiranyakashyap knew that Sinhika (Holika) had a boon, whereby, she could enter the fire unscathed.
Prahlad, who kept chanting the name of Lord Naarayana all this while, came out unharmed, as the lord blessed him for his extreme devotion.
Thus, Holi derives its name from Holika. And, is celebrated as a festival of victory of good over evil.
Holi is also celebrated as the triumph of a devotee. As the legend depicts that anybody, howsoever strong, cannot harm a true devotee. And, those who dare torture a true devotee of god shall be reduced to ashes.
Also read story about Draupadi's strong faith in Shri Krishna: