Once upon a time, Surya, the Sun God married a charming and attractive princess named Sangna. They were blessed with twins in course of a year and the twins were called Yama and Yamuna and they grew up happily together. But soon after, their mother couldn’t bear the brilliance of their father and decided to leave for Earth.
But she left her exact replica and called her Chhaya so that Surya felt she was still there. However, the new mother of the kids was very unkind towards them and she instigated the Sun God to throw them of their abode. As a result, the Yamuna fell on the Earth and started flowing as river Yamuna, while Yama went to the underworld to become the King of Death.
Years pass and the siblings miss each other until one day when Yama decided to visit her sister. After receiving the news of her brother’s arrival the Yamuna prepared for a grand feast in Yama’s honor and since it was just two days after Diwali, her home was already decorated and illuminated with lights. Yama was overjoyed by her sister’s gesture and they spent a delightful evening with each other after a long gap that separated them.
When the time of Yama’s return to the underworld struck again, he turned to his sister and asked him what she wants from her brother for welcoming him so lovingly. So the Yamuna asked for a gift which entailed that on this particular day all brothers would remember their sisters and pay them a visit if possible while all sisters should pray and pledge for their brothers’ happiness and prosperity.
This is how the custom of Bhai-Dooj, as we know today, came into practice
Another popular belief centering the inception of Bhai-Dooj is that on this particular day, Sri Krishna returned to his sister after killing the Narakasura Demon Subhadra warmly welcomed him with the flowers, lamps and sweetmeats and marked his forehead with a holy spot that protected him.
Nowhere in the world is the relationship between brother and sister glorified as that of India and these ancient myths have really played a role in adding a sense of grandeur and warmth to the celebrations.