Raksha Bandhan is considered to be the biggest festival of the Hindus, that celebrates the pure bond of love between a brother and a sister. On the occasion of Rakshabandhan, more popularly known as Rakhi, a sister ties a Rakhi which is a pious thread, on the wrist of her brother. Her brother promises to guard and protect her, all through life and also to support her whenever she needs him. The festival of Rakhi is celebrated on the last day of Shravan, which is a month according to the Hindu Lunar Calendar.
The Hindu mythology explains the origin of Rakhi, much before the Bhavishya Puran.
As per the Bhavishya Puran, once the god of thunder and Lightning, Indra was challenged by the King of Demons, Bali.
Indra's wife, Sachi, much concerned about her husband, went to Lord Vishnu, to seek his help. Vishnu sympathized Sachi and gave her a thread, which he had blessed. He asked Sachi to make Indra wear the thread. She reached Indra and tied the thread around his wrist and prayed for his Victory. With the thread, Indra fought the battle and won it against Bali. After his victory, he returned to Amaravati. Therefore in the ancient times, women tied a sacred thread on the hands of men to protect them during war.