As the foreign exchange market is highly volatile, there is no particular time per se to buy or sell USD in India. However, in economic terms, you should buy USD in India when Rupee is trading at relatively stronger levels against the US dollar so that you need to pay a lesser amount in INR to buy USD. Similarly, if you are looking to sell USD then you should do it when USD to INR exchange rate is on the higher side, i.e INR trading at weaker levels so that you get more INR per US dollar.