Updated: Mar 2
The first thing to note about choosing when to go to Bali is that it doesn't have the traditional four seasons of Spring, Summer, Autumn and Winter. Given it's close proximity to the equator Bali remains constantly hot throughout the year.
It does however have a dry and a wet season. The dry season runs from April through to October with the number of tourists reaching their peak in July and August due to the high heat, dry spells and relatively lower humidity. The wet season runs from October to March. In my opinion, the wet season is still a fantastic time to visit Bali, particularly if you intend to travel around the island rather than exclusively sunbathe as the number of tourists is lower meaning lighter traffic, excellent deals on accommodation and shorter queuing at attractions. Whilst the wet season does bring rainy spells these are often heavy downpours which last a short period of time as well as sporadic thunderstorms. I most recently visited Bali for three weeks in January at the height of the wet season and it only rained heavily 3 times, the rest of the time remained sunny between 25 - 30 degrees Celsius.
In short, there is no bad time to visit Bali! It's an amazing place to visit at any time of the year. So you've chosen Bali for your next adventure, you know when you're going now you just need to get some tips on what to bring!