According to one mythological story Gods lead by lord Indra, were on verge of defeat by
demon King Brutra. The king of Gods, Indra asked for advice from Guru Brihaspati to handle the situation. Brihaspati asked Indra to tie a holy thread on his wrist. The thread was powered by the sanctified mantras on the Shravan Purnima. Indrani tied the thread on Indra’s wrist and wished him to win. Since then it is a tradition to tie a knot on brother’s wrist.
It is assumed that the Raksha Bandhan was a followed by Yamuna and his brother Lord Yama (the Lord of Death). Yamuna once tied rakhi to Yama and this feeling moved Yama so much that he declared that whoever gets a rakhi tied by his sister and pledge her to protect her will become immortal.
These stories on Raksha Bandhan make the importance of festival more significant. They conveys the hidden message of the festival in its true form and help to strengthen the bond of brother and sister.