The story of Chaturmas, the period of the four months of monsoon, is narrated in the Bhagavat Puran.
Whoever listens to this story of the great bhakta Bali, and observes austerities and offers extra devotion during Chaturmas will earn the blessings of Vishnu Bhagwan.
The asuras and devas, bitter enemies though they may be, were brothers. Their father was Maharshi Kashyap and their mothers were his two wives, Diti and Aditi, respectively. Interestingly, they were both sisters, the daughters of Tvastr Prajapati, who was one of the mind-born sons of Brahmaji. His other mind-born sons included Kardama, Daksha, etc., and they are called Prajapati because the actual work of creation was done by them. Thus another name of Brahmaji is Pitamaha or Grandfather as creation was the work of his children.
Because their mothers were Diti and Aditi respectively, the asuras are called Daitya and the devas are called Adityas (especially Suryadev, who is the oldest.) The difference between the two can also be explained by their mothers’ names. Diti is what is finite and Aditi is infinity. It is described in the Chandogya Upanishad and elsewhere that both Indra, king of the devas, and Virochana, king of the asuras, had an opportunity to study brahmavidya. But Virochana thought it meant “There is nothing but the body so one can do whatever one likes.” Indra, however, grasped its true meaning that the self is beyond the limits of the petty whims and desires of the mortal body. For this reason, Bhagwan favours the devas over the asuras.
However, the good guys don’t always stay good and the bad guys don’t always stay bad. Sitting in Amaravati, his palace in Swarga, Indra became puffed up with pride and forgot the true source of his power – God. Meanwhile in Narak, Bali, son of Virochana, pondered over the error his father had made and resolved to purify himself of all evil by performing rigorous penance. As a result, the asuras were able to overcome the devas and Indra was dethroned.
Accompanied by other devas, Indra went to Vishnu Bhagwan and begged him to save them. Bhagwan, who is ever merciful to his devotees, agreed. He took the form of Vaman, an 8-year-old Brahmin boy. Vaman went to see Bali who received him cordially and offered him a gift. So, Vaman asked him for three steps of land. Looking at the little legs of the boy in front of him, Bali burst out laughing and urged him to ask for more. But Vaman was adamant so Bali granted the strange request. Immediately, Vaman grew to a colossal size. With one stride he covered the earth and with a second, the heavens. Laughingly, he asked Bali, “I have covered everything and still have one step to go. Where shall I put it?” Bali realized who he was dealing with and immediately offered up his own head for the third step. This is the sign of true nobility. When he was left with nothing Bali gave up his own head rather than break his word. Vaman took the third step and crushed Bali back down to Narak again.
Pleased with this display of devotion by Bali, Vishnu Bhagwan went to visit him and offered him a wish. Bali requested, “As I provided the third step, I request that you together with your wife, Lakshmiji, should come and live with me for one third of the year.” Vishnu Bhagwan agreed and since then devout Hindus have been observing various austerities and offering extra devotion during Chaturmas. During this time, Vishnu Bhagwan is considered to be asleep. Thus the start of Chaturmas is called Devshayani Ekadashi and the day he wakes up and Chaturmas ends is called Devprabodhini Ekadashi.