The રાહુ vrat is kept on શનિવાર — traditionally one meal after sunset, without salt. It is the household remedy for a weak રાહુ, 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.
Rahu has no fasting day of its own. Some traditions borrow Saturday from Saturn, but eclipse-day observance is the more direct remedy.
An afflicted Rahu shows as anxiety, addiction, scandal and chasing what recedes as you approach it. Durga and Bhairava worship, and eclipse-day practice, are the classical response.
શનિવાર. Fasting, charity and mantra japa for રાહુ all belong to that day.
The mantra, the weekday fast and the charity are safe for anyone — they address રાહુ without amplifying it. ગોમેદ is not: a planetary gem strengthens whatever the graha is already doing, so it needs a chart reading first.