How the date is fixed
Devutthana Ekadashi falls on the eleventh day of the bright half of the lunar month Kartika (Shukla Ekadashi). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Devutthana Ekadashi is the eleventh tithi of the bright half of Kartika, in November, the eleventh tithi after the Diwali new moon. It closes the four-month chaturmasa that began on Devshayani Ekadashi, and from this day the muhurtas for weddings resume. The Ekadashi rule applies: the tithi must be present at sunrise and the smarta and Vaishnava calendars can differ by a day. In Maharashtra it is Kartiki Ekadashi, the second great day of the Pandharpur pilgrimage, and Tulsi Vivah is kept on this day or the next.
How it is observed
The Ekadashi fast, the ritual waking of Vishnu at dusk with conch and bell, and the release of chaturmasa vows.