When Bali selflessly gave everything that Vishnu had asked for disguised as Vamana, Vishnu became very impressed by Bali’s devotion. He blessed Bali to be equal in status to Indra for as long as he lived. He also promised to protect Bali and his homestead and disguised himself as a gatekeeper guarding Bali’s palace. However, Vishnu’s wife, goddess Lakshmi, missed her husband in Vaikunth. Unable to bear her husband’s absence, she disguised herself as a poor Brahmin lady and went to Bali, and told him that she wanted a place to stay until her husband returned from the task he had set out to accomplish. Bali wholeheartedly welcomed her and protected her just like an elder brother.
With the arrival of the goddess, Bali’s palace was filled with happiness and wealth. On the day of Shravana Purnima, the poor Brahmin lady tied a coloured cotton thread around his wrist. Overwhelmed by the gesture, Bali granted her a wish. The Brahmin lady pointed at the gatekeeper and asked for her husband to be set free. In an instant, Vishnu and Lakshmi then returned to their true form. King Bali was touched by the love and care they had shown him and requested Vishnu to go back to his abode with Lakshmi. However, he requested Vishnu to visit him at least once every year. It is believed that lord Vishnu visits King Bali for four months every monsoon.
Source: Amar Chitra Katha
Painting by Amrapali