The Ketu vrat is kept on Tuesday — traditionally one meal after sunset, without salt. It is the household remedy for a weak Ketu, 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.
Ketu has no fasting day of its own. Ekadashi fasting supports the detachment Ketu asks for, and eclipse-day observance is the direct remedy.
An afflicted Ketu reads as sudden loss of interest, confusion about direction, and withdrawal from what one had built. Ganesha and Bhairava worship, feeding dogs, and steady practice are the classical response.
Tuesday. Fasting, charity and mantra japa for Ketu all belong to that day.
The mantra, the weekday fast and the charity are safe for anyone — they address Ketu without amplifying it. Cat’s Eye is not: a planetary gem strengthens whatever the graha is already doing, so it needs a chart reading first.