Krishna Chaturthi is the nineteenth tithi overall, the fourth day of the dark fortnight, and like its bright-fortnight counterpart it is sacred to Ganesha. Sankashti Chaturthi, kept on this waning-side fourth tithi each month, is a fast dedicated to Ganesha as the remover of obstacles and difficulties, the name means the fourth that removes distress, and is broken after the moonrise sighting. In the fivefold grouping the fourth tithi is Rikta, generally unfavourable for ordinary beginnings, but the Ganesha observance overrides that for its devotees. The waning Moon rises late in the night by this tithi. These are the conventions of the calendar and festival tradition, in which the fourth day of both fortnights belongs to Ganesha.
Sankashti Chaturthi is calculated by a rule distinct from most other festival tithis: it is fixed not to sunrise or midday but to whichever date the fourth tithi is prevailing at moonrise, since the fast is broken only upon sighting the risen Moon, so almanacs must compute the specific hour the Moon crosses the horizon on the relevant evening, a calculation that varies by latitude and longitude and is one of the more technically demanding routine determinations panchanga makers perform each month. Among the twelve monthly Sankashti Chaturthis, the one falling in the month of Magha, called Sakat Chauth in North Indian practice, is regarded as the most important of the year.