Significance
Holika Dahan, on the eve of Holi, celebrates the triumph of devotion over evil. It recalls how the demoness Holika, who tried to burn the young devotee Prahlad, was herself consumed by the fire while Prahlad, protected by Lord Vishnu, emerged unharmed.
The bonfire marks the burning away of negativity and the arrival of spring.
Rituals
A pyre of wood and cow-dung cakes is lit at the auspicious pradosh muhurat after sunset. People circumambulate it, offer grains, coconut and the season's harvest, and pray for prosperity. Ash from the fire is considered sacred.
How the date is fixed
The bonfire is lit on Phalguna Purnima during pradosh kaal (dusk), while avoiding Bhadra. The muhurat above is the Bhadra-free evening window for your location; Holi is played the next day.