The legend in the Bhavishya Puran
The legend refers to a war between the Gods and the Demons. The demon King Brutra was advancing and the Gods lead by lord Indra, were on verge of defeat. The king of Gods, Indra approached Guru Brihaspati to find a solution to the situation. Brihaspati asked Indra to tie a sacred thread on his wrist, powered by the sacred mantras on the Shravan Purnima. Lord Indra's Queen Sachi also called Indrani, empowered the thread and tied it on to his hand on the decided day. The power of the sacred thread called Raksha helped the Gods to victory.

The tradition of thread tying still continues. It is a gesture of goodwill.