Step 1: Recall significance.
National Fish Farmers Day is celebrated in India to honor fish farmers and recognize their contribution to aquaculture and fisheries.
Step 2: Historical reason.
The date 10 July marks the success of induced breeding of Indian major carps (Rohu, Catla, Mrigal) achieved by Dr. Hiralal Chaudhary and Dr. Alikunhi in 1957.
Step 3: Conclusion.
Hence, National Fish Farmers Day is observed on 10 July.
Final Answer: \[ \boxed{\text{National Fish Farmers Day is celebrated on 10 July.}} \]