Masik Durgashtami is the Shukla Ashtami of every lunar month - the eighth tithi of the waxing fortnight - kept in worship of Durga. It is a day-long observance, so it is dated by the tithi prevailing at sunrise rather than by an evening or night window. This calendar lists the upcoming dates for your city, and every row carries the rule that chose the day and the text that rule comes from.
Not every vrat is dated the same way. A vrat whose worship belongs to a particular division of the day - the evening for Pradosh, the middle of the night for Shivaratri, moonrise for Sankashti - is dated by the day on which the tithi fills that division. Durgashtami is not one of those: it is kept through the day.
So the rule that applies is the one Kane, History of Dharmasastra V.i p.73, gives for exactly that case - religious acts prescribed for the gods by day are begun in the morning even if the tithi is mixed with another on that day - and the day carrying the sunrise inside the Shukla Ashtami is the day shown. Every row on this page names that rule and its source rather than asking you to take the date on trust.
An Ashtami can hold two consecutive sunrises. When it does, the listing takes the earlier of the two days and says so on the row.
Kane, History of Dharmasastra V.i p.72, records that the general yugmavakya pairing rule is riddled with exceptions and names four overrides. One of them puts the 8th among the tithis taken paraviddha - joined with the tithi that follows it - which on a doubled Ashtami points at the LATER day. That override is not applied by the date engine, so on a doubled Ashtami an almanac following Kane's exception may print the day after this one. It is named here rather than left for a reader to discover as an unexplained discrepancy.
The monthly Durgashtami is the same tithi that, in the Ashvina Navaratra, becomes Maha Ashtami - the great eighth day of the autumn festival. The monthly observance is the ordinary recurrence of it; the Navaratra one is the annual peak, and it is dated inside the Navaratra schedule rather than on its own.
This page dates the monthly tithi only. The festival calendar handles Navaratra, which the engine deliberately renders as several competing Tithitattva readings rather than one, because the sources give more than one start.
Masik Durgashtami is the Shukla Ashtami - the eighth tithi of the waxing fortnight - kept every lunar month in worship of Durga. It is a day-long observance.
It is the same tithi. Maha Ashtami is the Ashtami of the Ashvina Navaratra specifically, dated inside that festival's own schedule; Masik Durgashtami is the ordinary monthly recurrence of the tithi.
When the Ashtami holds two sunrises, this listing takes the earlier day. Kane, History of Dharmasastra V.i p.72, names the 8th among the tithis taken paraviddha - joined with the tithi that follows - which points at the later day. That override is not applied here, and the page says so rather than hiding the difference.
No. Only a city, so the sunrise each tithi is read at belongs to your location.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.