Journal
The best time of year to photograph Bali
There is no single best time to photograph Bali. There is a dry season and a wet season, a ceremonial calendar that ignores both, and a tourist calendar that ignores everything — and which of those matters most depends entirely on what you are trying to photograph.
What follows is the honest version rather than the one that sells trips.

The two seasons
Dry season: April to October. Lower humidity, reliable mornings, hard clean light, and the sunrise-to-blue-sky sequence that most landscape photography of Bali depends on. It is also hotter in the sense that there is nothing between you and the sun by nine in the morning.
Wet season: November to March. Humidity around ninety per cent, rain most afternoons, occasional multi-day systems, and cloud that turns the whole sky into a soft box. Fewer people, lower prices, greener landscape, and behaviour — sheltering, waiting, improvising — that does not exist the rest of the year.
The transitions, roughly April and October, are the quiet compromise: some of the wet season's cloud and drama, most of the dry season's reliability.

Month by month
January–February. Peak wet. Heaviest rain, most flooding, fewest visitors outside the New Year fortnight. Hardest conditions and the most distinctive pictures. See photographing Bali in the wet season.
March. Rain easing. Nyepi usually falls here, which means the ogoh-ogoh parades the night before — one of the great photographic events in Asia — followed by a day when photography is not merely discouraged but effectively prohibited.
April–May. The shoulder, and arguably the best all-round window. Drying out, still green, cloud still doing interesting things, and the July crowds have not arrived. This is when the workshop runs, for exactly these reasons.
June. Reliable dry weather, comfortable temperatures, building crowds.
July–August. Peak dry and peak tourism simultaneously. Best light, worst crowds, highest prices. Every famous location has a queue. Clove harvest in the highlands begins.
September. Dry, hot, dusty. Crowds thinning. The landscape is at its most parched, which is a look — just not the look most people come for.
October. Transition back. First rains, dramatic skies, quiet. Underrated.
November–December. Wet season returns. Christmas and New Year bring a short, sharp crowd surge that has nothing to do with the weather.

The calendar that actually matters
All of the above is about weather, and weather is the less important variable if your subject is people.
Balinese ceremonial life runs on a 210-day pawukon cycle and a lunar saka calendar, neither of which has any relationship to the Gregorian months above. Galungan moves about seventy days earlier each year. Every temple on the island — and there are tens of thousands — holds an anniversary every 210 days, which means something is happening somewhere every single day of the year.
The practical consequence is liberating: you do not need to time a trip around a festival. You need to know how to find out what is happening in the week you are already there. That is covered properly in the Balinese ceremony calendar.
The two fixed points worth planning around are full moon and new moon — purnama and tilem — which are ceremonial days twice a month, entirely predictable, and lookupable from anywhere in the world.
Choosing by subject
Landscape and temples: May, June, September. Clean light, manageable crowds.
Street and documentary: the wet season, or the shoulders. Weather creates behaviour, and behaviour is the subject.
Ceremony: irrelevant. Check the pawukon for whenever you can travel and plan around what you find.
Portraits: wet season or any overcast day. The soft light under cloud is better on faces than anything the dry season offers.
Avoiding people: February, or September through early October.
The unhelpful but honest answer
The best time to photograph Bali is whenever you can go twice. A week in the dry season and a week in the wet will teach you more about the place than two weeks in either, because the difference between them is where the island's actual character shows up. Almost nobody does this, which is one of the reasons most photography of Bali looks the same — it is all made in July.