How the date is fixed
The eleventh day of each half of the lunar month, kept on the day Ekadashi is running at sunrise. If it starts and ends between two sunrises, the fast moves to the next day. Each Ekadashi is named after its North Indian (purnimanta) month; dark-half Ekadashis also show the amanta month, which is one month behind.
About
Ekadashi occurs twice a lunar month, on the eleventh tithi of the bright and the dark half, giving 24 in a normal year and 26 in a year with an adhika masa; each has its own name and story. The day is fixed by the tithi at sunrise, with a further condition: if Dashami is still running at the start of the day (in the smarta rule, at dawn; in the Vaishnava rule, at the 96-minute arunodaya before sunrise), the fast moves to the next day. This is why two almanacs can print consecutive dates for the same Ekadashi. The fast is broken (parana) in Dwadashi the next morning, after sunrise and before Dwadashi ends.
How it is observed
A fast from grains for the day, Vishnu worship, and the parana in Dwadashi the following morning.