Rakhsha Bandhan celebrates the pure bond of a brother and sister where the brother vows to protect his sister forever.
But it was actually not a sister who tied rakhi to her brother but a wife who tied the rakhi to her husband? Shocking, isn’t it?

But this is the reality which most of us don’t know. This story is about lord Indra and his wife Indrani. In the Vedic time, a war was fought between the dieties and demons on a full moon night (purnima) in the month of Shravana. The dieties were losing the war as the demons were stronger. Lord Indra feared that if they lose the battle, the earth will see the worst days and might enter into darkness. His wife, Indrani was equally worried.
Indrani was a very religious woman. So she prepared a amulet and tied it around Indra’s wrist to protect him. With the amulet on his wrist, Indra not only won the battle but also remained unhurt.
Therefore, since that day, the amulet is known as Raksha Sutra and that’s the reason why Raksha Bandhan is celebrated on the full moon of Shravan month.

But, even though rakhi was first tied by a wife to her husband, it later changed in the Vedic Period only.
This story is about Yama and Yamuna, children of Lord Surya (Sun). Lord Surya was married to Samjna. Samjna was unable to bear the strong radiations of Surya, so she decided to flee to her paternal home. But before fleeing, she left her shadow ( an another form of her) named Chaaya so that Surya doesn’t find out.

Eventually Chaya and Suryahad had a son later. But Chaaya was very cruel to her step son and step daughter, Yama and Yamuna. One day Yama came to know about her truth and both the brother and sister decided to revolt against her. Unfortunately, Chaaya got to know that Yama knew about her truth. Therefore, she cursed Yama with death and his body’s mutilation thereafter.
The curse terrified Yama and Yamuna as they were all mortal.
So Yamuna tied a Raksha Sutra or rakhi around her brother’s wrist. Because of Chhaya’s curse Yama met his unfortunate mutilation but the Raksha Sutra saved him from death.

That’s the reason why Yama is the god of death.
There are many such other stories related to the Raksha Sutra/ Rakhsha Bandhan like the story of Maharani Karnavati and Humayun, Laxmi and Bali, Porus and King Alexander etc. which depicts the true value and importance of Raksha Bandhan.