Mercury daan is given on Wednesday: green gram (moong), green cloth, books, pens and stationery. Charity addresses a graha without amplifying it, which is why it is prescribed where a gemstone is withheld.
Daan is charity aimed at a specific graha, given on its day and matched to its significations. Classically it is the remedy that works when a gemstone would be dangerous — it addresses the planet without amplifying it, which is why it is prescribed for Rahu and Saturn where stones are withheld.
Mercury charity is books, stationery and green moong, given on Wednesday to students, schools and libraries.
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.