During the winter and spring months there's an immense amount of rainfall distributed through the entire basin, mainly because of it is the wet phase of the distinct South American Monsoon System. The driest months happen during the summer and autumn months, when most part of the basin suffers with a considerable lack of rain through many days - especially the southernmost parts of the Basin. That's when the fire season happens, including the recent one which shocked
the entire world due to the extent and the organization of the fires - most of them were criminal acts against nature and humanity itself.
Many interesting things happen during the wet season, such as the presence of the so called Flying Rivers. These Flying Rivers are so huge they carry moisture and rainfall from the Tropical Atlantic to the southern region of Brazil and Argentina. Of course, moisture from the Amazon Basin also serves as fuel to these structures in the atmosphere.
There is also a huge difference in seasonality between the northernmost and southernmost parts of the Basin. In the extreme-north it is possible to see the occurence of rainfall during most of the year - quite like no seasonality at all - while the contrast between wet and dry in more evident in the southern region. The extreme-north - a region where is often called the Dog's Head because of its shape in the brazilian territory - is possibly the wettest region in the entire South American continent.
The trouble with the wet season in the Basin is that ENSO activity also gets its more mature phase quite at the same time at winter. That is why ENSO events are important for the region, because they can either disrupt (in the case of El Niño) or amplify the amount of rainfall (during La Niña)
expected for the season.