The short answer is: when the water temps are somewhere between 62 and 80 degrees Fahrenheit…give or take. While water temp is important other factors likely play a part as well. A study on carp spawning in Ontario found that even with different water temperatures carp spawned around the same time over two years. This suggests that so long as the water temperature is within the right range the fish will spawn around the same time each year – which is good news for anglers!
It’s worth noting that even in a scientific paper about carp spawning the authors noted that the fish in the area “began to feed intensively during the prespawning period” – so make sure not to miss one of the best times of the year to catch them!
The paper used for this article was: Swee, U. Boon, and Hugh R. McCrimmon. “Reproductive biology of the carp, Cyprinus carpio L., in Lake St. Lawrence, Ontario.” Transactions of the American Fisheries Society 95.4 (1966): 372-380.