How the date is fixed
Nirjala Ekadashi falls on the eleventh day of the bright half of the lunar month Jyeshtha (Shukla Ekadashi). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Nirjala Ekadashi is the eleventh tithi of the bright half of Jyeshtha, in late May or June, at the height of the north Indian summer. The Puranas tell that Bhima, unable to fast twice a month, was told to keep this one Ekadashi without even water. As with all Ekadashis, the day is chosen so that the tithi is present at sunrise and free of Dashami at dawn (smarta rule), with the Vaishnava rule sometimes giving the following day. The parana, or breaking of the fast, is done in Dwadashi after sunrise the next morning.
How it is observed
A complete fast without water from sunrise to the next morning's parana, and charity of water-pots and fans.