Jul/Aug or Dec/Jan for a trip to Iguazú Falls?
The larger-than-life Iguazú Falls are spell-binding at any time. You can visit all year round, so your decision should probably also be tempered by the ideal time for the rest of your holiday in Brazil and/or Argentina.
The falls are situated in a sub-tropical latitude so temperatures tend to be elevated all year. The seasons in the southern hemisphere are of course reversed, so the months of December/January fall in the summer, when humidity can be high and daytime temperatures reach well over 30°C. The sun is at its strongest then too. This the period of heaviest rain so the falls are usually at their fullest then (although the water flow can be partially controlled by dams upriver). At times of heavy rain some of the walkways may be closed.
In July/August temperatures are lower (mid-20s°C), it is less humid and there is less rain: this means the sight of the falls is a little less dramatic as the water breaks up into a necklace of smaller falls, but it is no less beautiful for that. Late December/January and July are both holiday months locally so there may be an increase in visitor numbers during those months. August may see a surge in international visitors. Early December might be the best time to avoid the crowds.