How the date is fixed
Devshayani Ekadashi falls on the eleventh day of the bright half of the lunar month Ashadha (Shukla Ekadashi). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Devshayani Ekadashi is the eleventh tithi of the bright half of Ashadha, in June or July. The Puranas describe Vishnu retiring to rest on this day and waking on Devutthana Ekadashi in Kartika; the four lunar months between are chaturmasa, during which weddings and other samskaras are traditionally not held. The Ekadashi rule applies: the tithi must be present at sunrise, and the Vaishnava and smarta calendars can differ by a day. In Maharashtra the Pandharpur wari of the Varkari pilgrims ends on this day.
How it is observed
The Ekadashi fast, Vishnu worship, and the taking of chaturmasa vows that run until Kartika.