Ekadashi is the eleventh tithi of each fortnight and one of the most important fasting days for followers of Lord Vishnu, falling twice in every lunar month - once in the waxing Shukla paksha and once in the waning Krishna paksha. There are two rules for fixing the date, not one, so this calendar lists every upcoming Ekadashi twice: once by the Smarta reckoning and once by the Vaishnava one, each with the Dwadashi parana window in which the fast is broken, computed for your location. No birth details are needed.
Ekadashi means "the eleventh" and marks the eleventh tithi counted from each new moon (Amavasya) and each full moon (Purnima). Because there are two fortnights in a lunar month, every month brings a Shukla paksha Ekadashi and a Krishna paksha Ekadashi, each traditionally dedicated to Lord Vishnu.
There are two rules, not one. The Smarta reckoning takes the day whose sunrise falls inside the Ekadashi tithi - the Udaya Tithi. The Vaishnava reckoning applies a second test as well, arunodaya-vedha: if Dashami is still running in the last four ghatikas (a fixed 96 minutes) before that sunrise, the Ekadashi is rejected and the fast moves to the next day. The distinction is set out in Kane, History of Dharmasastra Vol. V Part I, pp. 113 to 115. This calendar shows both dates rather than picking one, and says which rule produced each.
Because both rules are anchored to sunrise, and sunrise moves with your longitude and latitude, the calendar date of an Ekadashi can differ from one city to another. That is why the tithi boundaries here are computed for the location you choose.
Devotees fast on Ekadashi and break it the next morning on Dwadashi during the Parana window, after sunrise and before the tithi ends. The strictness of the fast varies by tradition and by the person:
How strictly the fast is kept is a matter of tradition and of what a person can manage, and no rule in this calendar depends on it. The dates and the parana window are the same whichever form you keep, so choose the one your family or your health allows.
The parana is not a loose "break it next day" instruction, and this calendar renders the rule rather than a rule of thumb. Kane V.i p. 120, citing the Kurma Purana, is explicit: the fast is on Ekadashi and the parana on Dvadashi, in the morning, and it must not be deferred to Trayodashi, "that would destroy the merit of twelve dvadasis". Because the sources give two readings of "morning", each date shows both windows - the first three muhurtas of the day, and everything before midday - along with the instant Dvadashi ends, which is the hard outer bound. Kane allows the thirteenth in exactly one case: when no Dvadashi at all exists on the day after the fast, and the calendar labels that case when it occurs.
Three things this calendar does not compute, and says so instead of guessing. Hari Vasara, the first quarter of Dvadashi that modern almanacs avoid, has no classical locus - the term does not occur anywhere in Kane Vol V Part I, including the Ekadashi chapter and the parana discussion - so it is reported as a convention and left uncomputed. The four Brahmavaivarta vedhas of Ekadashi that Kane names and then expressly passes over are not reconstructible from their names alone. And a third axis exists, orthogonal to Smarta and Vaishnava: on a doubled Ekadashi, Narada Purana purvardha 29.45 gives the earlier day to householders and the later to ascetics, while Kane p. 113 notes that sannyasins and widows follow the Vaishnava reckoning. That axis is not emitted as a separate date.
There are normally 24 Ekadashis in a year - two in each of the twelve lunar months, one in Shukla paksha and one in Krishna paksha. In a year with an Adhik Maas (extra leap month), this rises to 26.
Because there are genuinely two rules. The Smarta date is the day whose sunrise falls inside the Ekadashi tithi. The Vaishnava date applies arunodaya-vedha as well: if Dashami is still running in the last 96 minutes before that sunrise, that Ekadashi is set aside and the fast moves to the following day, which is why the two dates separate several times a year. Sunrise itself also shifts with your city, which moves both dates together.
The fast is broken the next morning on Dwadashi, after sunrise and before the Dwadashi tithi ends - never deferred to Trayodashi. Each date here shows that window under both classical readings of "morning": the first three muhurtas of the day, and everything before midday. In the one case where Dwadashi has already ended before sunrise, there is no Dwadashi left on that day and the parana is marked as falling on Trayodashi.
It is the extra test the Vaishnava reckoning applies. Arunodaya is the last four ghatikas before sunrise, taken here as a fixed 96 clock minutes rather than a proportional muhurta. If the Dashami tithi is still running inside that window, the Ekadashi is considered touched by the tenth and is rejected, so the Vaishnava fast moves to the next day. Kane V.i pp. 113 to 115 sets out the rule; Narada Purana purvardha 29.39 to 40 states the principle behind it, that "all should accept ekadasi free from dasami".
Follow your own tradition, which is why this calendar prints both instead of choosing. Smarta households take the sunrise-tithi date; Vaishnava practice takes the arunodaya-vedha date, and Kane V.i p. 113 records that sannyasins and widows follow the Vaishnava reckoning. Each entry here is labelled with the rule that produced it, so you can pick the one your family keeps.
No. Ekadashi is a calendar observance, not a birth-chart calculation. Only your city is needed, because both rules are anchored to sunrise and sunrise depends on your latitude, longitude and time zone - which is also why the date can differ between two cities on the same day.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.