Bali has a tropical climate appropriate to its proximity to the equator. Year round temperatures averaging 31 degrees Celsius. High humidity can be expected during the Wet Season between the months of October - April. The Dry Season between the months of May - September have also the lowest humidity.
The Wet Season brings daily rain and quiet overcast days with the most rain recorded between December - February. Occasionally rainfall can also be expected during the dry season but usually at night or very early morning. June - August there is usually a very refreshing cool breeze all day long. The central mountain area is typically cooler than the lower coastal areas mainly especially at night.
The best time to visit Bali is between June to September, the raining day will be hard to find. :)