Although there are various legends of Raksha Bandhan, but the legend mentioned in Bhavishya Purana is the most authenticated one. The legend of Bhavishya Purana is also mentioned in Vratraj behind the ritual of Raksha Bandhan.
Ages ago, there was intense battle between Gods and Demons. The intense battle continued for twelve years. At last Gods lost the battle and Demons conquered the kingdom of Indra along with all three Loks.
The God of Gods, Indra consulted Brihaspati, the Guru of Gods. Guru Brihaspati suggested Raksha Vidhan to Indra along with the Mantra to perform it.
On the day of Shravana Purnima Guru performed the ritual of Raksha Vidhan. During Raksha Vidhan, the Raksha Potli was fortified with sacred Mantra. After Puja, Shuchi the wife of Indra tied the Raksha Potli to the right hand of Indra.
Due to power of Raksha Potli, Indra was able to defeat Demons and get back His lost kingdom. Since then the ritual of Raksha Bandhan is performed during Sharvana Purnima.