Skanda Sashti is kept on the Shukla Shashthi - the sixth tithi of the waxing fortnight - and so recurs once every lunar month. It is dedicated to Skanda, also called Kartikeya, Subrahmanya and Murugan. This calendar lists the upcoming Shukla Shashthi dates for your city with the exact moment each tithi ends. Shashthi is one of the tithis the digests treat specially, and the page below says exactly which reading the list applies and which refinement it leaves out.
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Every row here is a civil day whose sunrise falls inside the Shukla Shashthi. That is the rule Kane, History of Dharmasastra V.i p.73, gives for an observance kept through the day: religious acts prescribed for the gods by day are begun in the morning even if the tithi is mixed with another on that day.
The dates come from this site's own almanac engine, which computes the tithi boundaries from the true Sun-Moon elongation to sub-arcsecond precision for the date and city you choose, rather than from a printed table. What the list does not do is apply Shashthi's own vedha, which is described next.
Vedha means contact: the earlier tithi persisting into the day and thereby vitiating the later one. The general magnitude is six ghatikas, but a handful of tithis are given a larger one. Kane, History of Dharmasastra V.i p.73, makes Panchami's contact with Shashthi twelve ghatikas, and the Dharma Sindhu digest states the same figure in almost the same words: Shashthi's Panchami vedha is of twelve ghadis.
A ghatika here is a fixed twenty-four minutes, not a proportional muhurta, so twelve ghatikas is four hours and forty-eight minutes. Where the Panchami runs that far past sunrise, an almanac that applies the override may print the following day instead. This page does not apply it, and says so rather than presenting an unqualified date.
A tithi can be long enough to hold two consecutive sunrises, which the almanac calls vriddhi. Both days are shown here, marked, rather than one being silently dropped.
The mirror case is kshaya: a Shashthi short enough that it begins after one sunrise and ends before the next, so no sunrise falls inside it at all. It is listed on the civil day it runs in, marked, with the tithi's span. The digests' express rules for a lost tithi all give that day and none drops the rite (Nirnayasindhu p.139; Dharmasindhu pp.9, 66, 69, 89), and the Skanda-vrata takes the Shashthi joined with the Panchami in any case (Dharmasindhu p.9, "षष्ठी स्कन्दव्रते पूर्वविद्धा"; Kane V.i p.76).
Kane, History of Dharmasastra V.i pp.75-76, records that the general yugmavakya rule is riddled with exceptions and names four overrides. One of them puts the 6th, the 8th, the Amavasya and the dark-fortnight 13th among the tithis taken paraviddha - joined with the tithi that follows. For a doubled Shashthi that points at the later of the two days, and the yugmavakya's own rewarding pair of the 6th with the 7th points the same way. The page states this on the row rather than resolving it for you.
The monthly Shukla Shashthi listed here is distinct from the six-day observance kept in the month of Kartika in the Tamil tradition, which runs from the Shukla Pratipada to the Shashthi and culminates in Surasamharam. This calendar dates the monthly tithi; it does not schedule the six-day festival.
The fast itself is commonly kept from sunrise and broken after the evening worship. Practice varies widely by region and by lineage, and this page deliberately makes no claim about what any particular observance obtains.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.