According to Hindu mythology, credited to Hindu scriptures Bhagavata Purana and Vishnu Purana, after lord Vishnu won the three worlds from the demon King Bali, he requested lord Vishnu to live in his palace. Lord Vishnu granted his wish. Lord Vishnu, being the greatest of all gods had to keep his promise hence he started living in Bali’s palace with his consort Lakshmi.
But Goddess Lakshmi did not like the palace or his new found friendship with Bali, and preferred that her husband and she return back to Vaikuntha.
Goddess Lakshmi disguised herself as a Brahmin woman and went to Bali to seek protection till her husband returns. Bali, kind by nature, welcomed her into his palace.
Goddess Lakshmi knowing the opportunity, came on the right occasion of Shravana Purnima. She tied a colored cotton amulet (Rakhi) on Bali’s wrist. Bali was greatly touched by this gesture of the Brahmin women (Lakshmi) and asked her for a gift. Goddess Lakshmi grabbed this opportunity with both hands and revealed her real identity and asked Bali to free her husband (lord Vishnu) from his request. As Bali had given his words, he had to relent but he extracted a promise from the couple to visit his palace every year on this day for a meal with him.
Inspired by this day, tradition has emerged when sisters visit their brothers on the day of Raksha Bandhan (Shravana Purnima) and have meal with them. On this auspicious day, they tie a thread (Rakhi) on the wrist of their brothers and wish for their long life and prosperity.