Pradosh Vrat is a fast dedicated to Lord Shiva, kept on Trayodashi - the thirteenth lunar day - of both fortnights, so it recurs twice in every lunar month. Worship is offered in Pradosh Kaal, which begins at sunset and runs for the first three of the night's fifteen muhurtas. Because it is an evening observance, the vrat falls on the day whose Pradosh Kaal the Trayodashi actually fills, which is often not the day it happens to be running at sunrise. This calendar lists the upcoming dates for your city with each one's weekday name and computed Pradosh Kaal window.
The vrat is anchored to the evening, not to sunrise, which is the single most important thing to understand about its date. Everything else follows from that:
Everything below follows from that evening anchor: it is why the window is noticeably longer in December than in June, why two cities can keep the same Pradosh on different dates, and why a panchang that lists tithis by sunrise will sometimes disagree with this calendar.
The weekday on which a Trayodashi falls gives that Pradosh its name. Because the vara runs sunrise to sunrise, the Pradosh Kaal window - which opens at sunset - belongs to the weekday of its own civil date. Popular practice, rather than any classical text we can point to, associates each weekday variant with a particular intention:
These intentions are not graded or computed anywhere in the calendar. It names the weekday variant and leaves the association alone, because no located text assigns a particular fruit to each weekday Pradosh - the list below is what practice reports, not what a chapter prescribes.
A Trayodashi can touch the Pradosh Kaal of two consecutive nights, and the rule for choosing between them is the reason this calendar exists in the form it does. The governing principle is Kane, History of Dharmasastra Vol. V Part I, p. 73: all vratas to be performed in the evening or night have to be performed on the tithi existing in the evening or night, even though it may be mixed up with another tithi. So the calendar first asks which night's Pradosh Kaal the Trayodashi actually pervades, and takes that night.
When it pervades exactly one, the choice is settled and the entry says so. When two nights both qualify, the calendar takes the one with the greater overlap - first of the Pradosh Kaal itself, then of the whole night arc - and labels that decision as a modern heuristic, because no classical text grades pervasion this way; it is stated only by present-day panchang makers, and it is used solely to make a genuine two-candidate case deterministic. The definition of Pradosh Kaal itself is flagged as digest-sourced rather than a verse: the Dharma Sindhu's Samanya Paribhasha glosses the nakta meal as taken three muhurtas after sunset, in the Pradosha kaala. At latitudes and dates where the Sun does not set at all, the window does not exist, and the entry says the reckoning was degraded rather than printing a fabricated time.
Pradosh Vrat is a fast dedicated to Lord Shiva observed on Trayodashi, the thirteenth lunar day of each fortnight, so it falls twice every lunar month. Devotees worship Shiva during Pradosh Kaal, the twilight period beginning at sunset.
Pradosh Kaal is the first three muhurtas of the night, beginning at sunset. Since the night is always divided into fifteen muhurtas, that is exactly one fifth of the time from sunset to the next sunrise - around two and a half hours at Delhi's latitude, longer in winter and shorter in summer. It is computed from your city's sunset, which is why the calendar asks for a location. The definition comes from the Dharma Sindhu's Samanya Paribhasha, which glosses the nakta meal as taken three muhurtas after sunset, in the Pradosha kaala.
A Pradosh takes its name from the weekday on which the Trayodashi falls - for example Soma Pradosh on Monday or Shani Pradosh on Saturday. Each weekday variant is traditionally kept for a particular blessing, such as Shani Pradosh for progeny and relief from Saturn's afflictions. The naming is popular usage rather than a rule from a located text.
Because Pradosh Vrat is an evening observance, not a day-long one. Kane, History of Dharmasastra V.i p. 73 states that vratas performed in the evening or night are kept on the tithi existing in the evening or night, even if it is mixed with another tithi. So the vrat falls on the day whose Pradosh Kaal the Trayodashi fills, which is often not the day the Trayodashi is running at sunrise - and a panchang that lists tithis by sunrise will therefore look one day out.
The calendar takes the night the Trayodashi genuinely pervades. When both nights qualify, it takes the one with the greater overlap - of Pradosh Kaal first, then of the whole night - and labels that as a modern heuristic rather than a verse, because no classical text grades pervasion by amount. The label is on the entry, so you can see exactly how that date was settled.
No. Pradosh Vrat is a calendar observance, so only your city is needed. It matters more here than on a sunrise-based calendar, because Pradosh Kaal is measured from your local sunset and is one fifth of your local night, so both the date and the window length change with your latitude and the season.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.