September and October are the best time to visit Foz do Iguacu as the city very rarely experiences rain then and temperatures remain comfortably pleasant with low humidity.
High Season (January and February): If you visit this city during this time, you can face a lot of crowds due to peak season and the local residents of Argentina and Brazil who come here for holiday. As this is the height of the summer, heat and humidity can be excessive. However the falls are quite impressive as the water level is high.
Easter: It has become a tradition in both Argentina and Brazil to visit Iguazu Falls during Easter which is also a good season to visit this place. The few days surrounding Easter Sunday can be even busier and more crowded than January and February.
Rain (May to July): The rainiest months are May and July. As a result the water is at its highest and the falls are quite spectacular. Usually the showers do not last long. The city is not as crowded during these two months and the prices for hotels are a bit lower due to off season in Foz do Iguacu. Just be sure to pack a rain poncho or umbrella.