There are many variations on this 2600 year old Chinese folk legend, including Japanese, Korean, Vietnamese and Indian versions, but here is the one I know.
Seven sister goddesses were traveling through the countryside. The day grew late and the lightning bugs began to appear, so the sisters stopped to rest and bathe in a secluded lake. As they talked and washed their long, shining hair, their voices carried through the mist, as sound sometimes does over water.
A man named Niulang lived alone nearby with only an old ox to keep him company. Strange voices were uncommon near his lonely home, so he wandered out to see what was the cause.
That was the first moment he saw Zhinu, the youngest of the sisters. When their eyes met through the evening mist, each felt a flame spark in their heart. Knowing that he had found his match and would never love another, Niulang immediately asked Zhinu to marry him, and she gladly agreed. Soon after, their two children were born.
Zhinu’s mother was a powerful goddess who lived in the heavens. When she found out that her daughter had married a human, a simple oxherd at that, she forced Zhinu to return home and resume her duties as the weaver of sunset clouds.
To Niulang, it was as if his beloved had disappeared. In the depths of his misery, the old ox that had kept Niulang company began to speak. The ox instructed the man to slay him and wear his hide in order to travel to the heavens. Niulang did so, with great pain, for he loved the old ox. When the deed was done, he wrapped himself and his children in the oxhide, and set out to travel to the heavens where Zhinu was.
But Zhinu’s mother was not to be spited. She removed her hairpin and drew it across the sky, creating a wide, silver river between Zhinu and Niulang—a river known to us as the Milky Way.
Zhinu’s six sisters had seen the love that she shared with Niulang, and asked their mother to permit the lovers to see each other once a year. Eventually, the powerful goddess relented, and once each year, the magpies fly up to the heavens and form a bridge between the stars Altair and Vega so that Niulang and Zhinu can spend one night in each others’ arms.
On this night, it rains until dawn, as the magpies weep tears that fall from the heavens for the bittersweet reunion.