Three years of drought
In the reign of Mandhata, the dharma-atma king of the Ikshvakus, the rains failed for three continuous years. The Sarayu shrank to a thread; the fields cracked; cattle collapsed at half-empty troughs; mothers held children whose lips had turned grey. Mandhata, who took every failure of his people upon himself, walked his own capital by night and heard the crying from every hut.
The king in the forest
Holding himself responsible, he laid aside the crown, took only a walking-staff and a lota, and left the palace to wander his own forests seeking answers from any rishi who might read the cause. For weeks he moved from ashram to ashram, sleeping under trees, sharing what little the tapasvis had, asking always the same question: 'What in my rule has offended dharma, that the sky itself has closed?'
Angira's guidance
At last, deep in a Vindhya vana, he came to the ashram of Rishi Angira. Angira looked upon the exhausted king and said with kindness, 'Rajan, in your vast kingdom a single low-born ascetic has undertaken tapas which the shastra does not permit him — the daily Vishnu-arpan of your praja has thereby been quietly disturbed. This is no fault of yours, but of the balance of karma. There is one remedy: the Devshayani Ekadashi of Ashadha Shukla is upon us. Tonight, at this very sunset, Bhagwan Vishnu enters his four-month yog-nidra. Observe the vrat with your whole kingdom — catch the last raise of his hand before his eyes close.'
The rains
Mandhata returned overnight and by dawn had sent riders to every village. The whole kingdom fasted together on Ashadha Shukla Ekadashi; every temple kept jagran through the night with Vishnu Sahasranama and Purushasukta; even the elephants of the royal stables were denied fodder in observance. Before the first light of Dwadashi, thick clouds rolled in from the ocean. By parana the rains had returned in full torrent, the Sarayu ran high, and the earth breathed again. From that year onward this vrat became the doorway into Chaturmas — the season when the earth herself is Vishnu's shayya.
"Grants prosperity, ends droughts and public calamities, and marks the start of the four-month Chaturmas sadhana period in which vratas performed carry many-fold their normal punya."