The Sun vrat is kept on Sunday — traditionally one meal after sunset, without salt. It is the household remedy for a weak Sun, and unlike a gemstone it cannot be kept against your chart.
The weekday fast is the household remedy — kept on the graha’s own day, usually one meal after sunset and no salt. It is as much a discipline as a ritual: the point is the weekly act of restraint, held long enough to mean something.
The Sunday fast is kept without salt, eaten once after sunset, and is the standard prescription for a weak Sun.
The corpus describes an ill-placed Sun as low self-esteem, absent energy and confidence, and friction with authority and with the father. That is the pattern these practices address.
Sunday. Fasting, charity and mantra japa for Sun all belong to that day.
The mantra, the weekday fast and the charity are safe for anyone — they address Sun without amplifying it. Ruby is not: a planetary gem strengthens whatever the graha is already doing, so it needs a chart reading first.