Deciding when to travel to Bali comes down to weather for most travelers. Since this is a tropical destination, Bali essentially has just two seasons—the dry season and the rainy season. The rainy season lasts from October to March, and in addition to being wetter than the dry season, it is also more humid.
Heavy downpours are common during the rainy season in Bali, and this is enough to dissuade some travelers from considering a rainy season visit. December through February is when the rainfall totals are most often at their highest, so this is an especially good time to avoid. At times, the rain is so heavy during the wet season that it can be hard to see anything at all when scanning the horizon. It can also rain for days on end, which can put a damper on things.
April through October is definitely the peak travel season in the Bali and Lombok area. This is not difficult to understand. The threat of heavy thunderstorms is much more limited than it is during the December to February period for starters. Also worth noting is the fact that refreshing breezes are much more common during the peak travel season, and much like the humidity, the overall temps are down.
Travelers can expect the Bali hotels to raise their rates during the peak, dry season. The holiday season (Christmas to New Year's), is when the rates tend to be at their highest. Due to the increased lodging rates during the peak travel season, budget travelers might focus first on the off season when trying to decide when to travel to Bali. That being said, vacation packages for many a world-renowned destination are easy to come by these days, so you never know what kind of deals you might turn up during the peak season.
Weather isn't the only thing to consider when trying to decide when to travel to Bali. Holidays and other annual events can also be worth keeping in mind. The annual festivals are particularly interesting and include both religious and non-religious celebrations. Among the most renowned festivals in Bali is Galungan. It is held every 210 days, and each installment of the Hindu affair lasts about a week and a half. Gods and spirits are honored during the Galungan Festival, and the various festivities include feasts and dances. On the last day, or Kuningan as it is known, locals dress in traditional clothing and carry various offerings to the area temples.
The Balinese New Year holiday of Nyepi is certainly not to be overlooked when it comes to planning a vacation in Indonesia. Held sometime between mid-March and mid-April (changes depending on the year), this one-day event sees locals celebrating the beginning of a new year in strange fashion. Everybody stays inside from 6 a.m. on "New Year's Day" until 6 a.m. the next day. In fact, guards patrol the streets so that people don't leave their homes, and the Bali hotels are not permitted to let their guests leave. Everyone remains silent on top of things, as this is believed to encourage evil spirits to abandon the island. Also worth highlighting when it comes to Nyepi is the fact that the airport is shut down. That means no flights, even if you aren't planning on observing the holiday.