Significance
Devshayani Ekadashi, also called Ashadhi Ekadashi, marks the day Lord Vishnu retires to his cosmic sleep (yoga nidra) on the serpent Shesha for four months. It begins Chaturmas, a period of austerity and devotion.
In Maharashtra it is the culmination of the Pandharpur Wari pilgrimage to Lord Vitthal.
Rituals
Devotees fast, keep vigil, and worship Lord Vishnu. During the ensuing Chaturmas, auspicious ceremonies like weddings are avoided, while spiritual practices, fasting and charity increase. Vitthal devotees throng Pandharpur.
How the date is fixed
It falls on the Ekadashi (11th tithi) of Ashadha Shukla paksha. Vishnu "awakens" four months later on Prabodhini (Devuthani) Ekadashi.