Bali only has 2 seasons, the wet season and the dry season. It's quite rare that you will see any much rain at all. Though on some occasions it can still rain on some afternoons for a short period of time. There is no winter, autumn, spring or summer in Bali. From my experience I have found that it seems to be warmer in the wet season in Bali.
Bali is located approximately 8 degrees south of the equator. The dry and wet seasons are typical for any country this close to the equator.
This season is typically between the months of May and October. During the peak of the season is when there is slightly higher temperatures than the start of the season. The average temperature range during this season is approximately 28° C - to 32° C (82.4° F - 89.4° F) however it can sometimes go outside this range.
This is the most popular time to visit Bali due to the consistent sunny days. If you plan to visit Bali during this period you will only need to take summer clothes with you. Maybe also take some light warm clothing in the event some evenings are a bit cooler. Most of the time though you will only need light summer clothing. It's also a great time to experience all the outdoor activities Bali has to offer. Such as outdoor adventure tours and water sports and activities.
It's quite often a lot more challenging to seek out Bali hotels and other accommodation options due to the increased demand from tourists. It's a good idea to plan way in advance for those planning to visit Bali during this time of the year.
I personally enjoy Bali more in the wet season. Although it does rain a bit more it never really rains for that long the times that it does rain. The main reason I like Bali more in the wet season is because it seems to be warmer weather than in the dry season. This is mostly because of the higher humidity.
I found that it was never too cold to swim during the wet season even during the evenings. On some days it does actually get quite hot when it's the wet season. It still does get hot during both of the seasons in Bali all year around however I could feel it more during the wet season. I enjoy swimming and nice warm weather so for me personally I prefer visiting Bali when it is the wet season. Although it is great weather all year around in Bali, there seems to be less cold nights and cool days during the wet seasons.