Holika Dahan, burning of old things, farm waste materials, woods or lighting of fire, is one of the most important ceremony during Holi festival. Holika Dahan 2014 date is March 16. This bonfire symbolically represents the victory of good over evil. On the night before Dhulhandi Holi, people light fire with old wood, sticks, branches and leaves.
The most popular legend associated with Holik Dahan is that of Demon Holika, sister of Hiranyakashipu, trying to kill young Prahlad and in that attempt gets herself killed. Young Prahlad is saved due to his unwavering and unfathomable devotion for God Sri Hari Vishnu. In some regions, people believe that Prahlad was saved by Lord Agni.
The day is observed as Hutashani Poornima in some regions. You can find more details about Holika Dahan and the story associated with it in this earlier article.