During the dry season which is between the months of April and September, Bali receives the most visitors and temperatures are if at all only a tiny bit higher than during the other months. During Bali's dry season one can still expect occasional rainfall though and sometimes even the locals are surprised how dry a rainy season can be and how rainy a dry season can be.
During rainy season which is typically from October to March Bali becomes more quiet. Bali's central mountain area is typically cooler and also more rainy than the lower coastal areas. Especially at night temparatures can drop to 15 degrees in certain areas higher up near the volcanos. The southern peninsula Bukith has less rain than the rest of south of Bali (Kuta, Legian, Seminyak, Denpasar).
Have a look at the climate table for Denpasar, which gives you an idea about high and low temperatures during the day and the expected rainfall during dry season and rainy season:
Bali's is located very near to the equator, therefore its warm, tropical climate makes it a huge draw for tourists. Average year-round temperature stands at around 30°C with a humidity level of about 85%.
Generally one can say that Bali is pleasant all year through. Even if it rains, Bali is enjoyable. Particularly for some surfers who enjoy big waves some months during rainy season are more suitable. The two seasons are not clear cut and fully predictable. At times, a week in November on Bali can feel like a June and the other way around.
Christmas and New Year is clearly high or peak season on Bali and tourists come from all over the world to enjoy the beaches, the surfing, diving, the restaurants, the club, the spas and everything else Bali has to offer - although it is technically "rainy season".
What to wear and AC on Bali
The high temperatures combined with the high humidity is not easy for some, as also during the night the temperatures do not drop too much. At the coast this is not too bad because of the nice and fresh breeze from the sea. Most tourists enjoy air conditioned hotel rooms or at least use fans to cool down the rooms. Not considering any dress codes for classy restaurants or dinner parties we suggest loose summer clothes such as shorts, t-shirts, wide blouses and thin linen trousers. Men can get around with decent shorts and a shirt almost everywhere, even in the expensive clubs. Women in dresses have no problems anywhere but in the temples, where long sleaves and a sarong must be worn. In a way it is always summer on Bali...and yes...sun protection is a must if you are not used to the intense sun of the tropics.
Weather on Bali
See above the average temperatures on Bali for January, February, March, April, May, June, July, August, September, October, November, December