The story
Zhīnǚ the weaver girl, seventh daughter of the Jade Emperor, came down to bathe and a cowherd took her clothes. She married him and they had two children, and when heaven found out she was taken back and the Milky Way was drawn across the sky between them.
The magpies took pity and make a bridge of themselves one night a year. The two stars are Vega and Altair, and the river between them is the galaxy; if it rains on the night, that is the two of them crying.
It was for most of its history a festival of the girls rather than of couples — a night to demonstrate needlework and ask for skill in it. The lovers' reading is what has survived into the shops.