Bali’s climate is like the rest of Indonesia which is tropical, thus only have two seasons which is dry season and rainy season. You may get sun all year round and during rainy season just more rain but normally in the evening, making it a good place to travel to anytime in the year.
Bali’s dry season is April to October, where you get lots of sun and it is less humid. But this is unfortunately the peak season so you will be swamped by crowds and resorts will charge higher rates.
The rainy season is worst from November to February, so you might consider to skip these months when planning to travel here.
As for Indonesia festivals, you may consider to come end of March for the Nyepi festival, it is unique whereby people do not work, talk or eat in a day. There is also another festival which is the Galungan festival.