@ aSHISH cHADHA,
Always follow your astrologer.
Secondly, actually grnet/hessonite should be worn on 1. saturday, 2. during the nakshatra of Rahu falling on Saturday 3. After sunset 4. In Rahu’s Hora (all these four combintions) should be met, but usually it is difficult to get the combination, also it takes time by the astrologer to get such a combination for you and as the astrologer is usually not paid enough to match his knowledge and the effort they the asrologers devised an easier way out by suggesting Saturday.
Wishing you good luck, may God bless you and above all?????
CHEERS