The story
A poor Brahmin widow named Shuchivrata lived by begging with her young son. One day, on the way home, she found a little boy of royal bearing alone and weeping. He was Dharmagupta, son of the king of Vidarbha, whose father had been killed in battle and whose kingdom had been seized by enemies. She took him home and raised him as her own.
One day the widow took both boys to a temple, where they met the sage Shandilya. The sage recognised the prince, told the widow his story, and taught all three the Pradosh vow: to fast on Trayodashi and worship Shiva in the twilight after sunset, when Shiva dances and all the gods attend him.
They kept the vow faithfully. Once, while bathing at a river, the widow’s son found a pot of gold, which he shared with his foster-brother. Another day Dharmagupta met Anshumati, a gandharva princess, in the forest; they fell in love, and her father, told by Shiva in a dream, gave her to him in marriage.
With the gandharva king’s army and Shiva’s grace, Dharmagupta defeated the usurpers and regained Vidarbha. He made the widow the queen mother and her son his chief minister, and all of them continued the Pradosh vow for the rest of their lives.
The Pradosh period recalls another tradition too: that Shiva drank the poison halahala that rose from the churning of the ocean at twilight on Trayodashi, and the gods praised him at that hour.
Retold in our own words from the traditional story; versions told in families and booklets differ in details.