The Mercury vrat is kept on Wednesday — traditionally one meal after sunset, without salt. It is the household remedy for a weak Mercury, 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 Wednesday fast is vegetarian and traditionally kept with green moong — the colour and the grain both belong to Mercury.
A weak Mercury shows as scattered thinking, careless speech, difficulty finishing what was started, and nervous restlessness. Wednesday practice and service to students are the classical response.
Wednesday. Fasting, charity and mantra japa for Mercury all belong to that day.
The mantra, the weekday fast and the charity are safe for anyone — they address Mercury without amplifying it. Emerald is not: a planetary gem strengthens whatever the graha is already doing, so it needs a chart reading first.