There is no best time as such to visit Curitiba. It is never hoarded with tourists and the weather is quite pleasant all year round. Although the winters get really chilly; the tropical climate is sure to be different for you than the usual four seasoned climate. It also lies in a temperate zone and hence the weather is somewhat like this. There is no defined Summer and Winter. Precipitation can happen at any time of the year and the last snowfall was in the year 2013.
January-April: During these months, the average temperature goes up to 30 degree Celsius and the lowest remains at 16 Degree Celsius. Even though, the temperature can go down to 8 Degree Celsius too, this season is still quite pleasant to travel in. The rainfall is the highest during this time.
May- September: This can be in a way described as the most typical climate for Curitiba with the overall feels of the weather. The temperature remains a comfortable 14 degree Celsius to 16 Degree Celsius. Although the temperature can go below 0 degree Celsius, most of the times a set of warm clothing suffice. Even during these months, be prepared for untimely rainfall.
October- December: October and November continue the streak of chilly weather in Curitiba and the weather starts to settle down a little in the month of December. The average during this time fluctuates from a 10 degree Celsius to a 15 Degree Celsius.