Petropolis’ Oceanic climate means that temperatures are mild throughout the year. You can visit any time and still enjoy much of the city’s charms. The climate is somewhat similar to Rio, though the higher elevation does make it cooler.
Ultimately the decision for best time to visit really comes down to whether you want to go when its lively, in July, or when its quiet. Weekends and Brazilian holidays sees people pour into town. If you’re only passing through, definitely don’t put Monday in your travel vacation itinerary planner, when almost everything fun is closed.