It is a hard question to answer. The weather forecast in the last 10 years is changing and making it difficult to know the best time. What I´m going to do is making a list of information from different sources to help people plan their trip.
This will help one to figure out about the four seasons we have in the southern hemisphere. Depends on the year dates will change.
Spring starts on September 21
Summer starts on December 21
Autumn starts on March 21
Winter starts on June 21
In 2020, in April and May the water level at the falls was low. It is not raining regularly as years ago. Commonly it would rain a lot in April and May and October, but in 2020 it was not happening. Depends on the year it can also rain a lot in May and June. Last time we lost the walkways at the falls was on June 8th, 2014, during a big flood. This year 2021, in April and May the water level at the falls was very low. June looks like a wet month. The water level is rising.
It can rain all year round, some months less or more.
October can be the wettest month.
August can be the driest month.
The average amount of annual precipitation in Iguazu Falls is around 2000 mm (78 in)
Our Summer is very humid and hot, the average temperature is 31º C (88º F)
Our Winter is not very cold, the average temperature is 17 º C (63º F)
The average year temperature is around 21º C (70º F)
In the winter mornings can be cold but in the afternoon it warms up.
70% of the days throughout the year are sunny.