WHEN to go?
The weather will never be ideal on all your stops. So, focus on what you want to do most and research conditions there: if a Himalaya trek is your highlight, don’t land in Nepal mid-monsoon; if you want to swim with whale sharks off Western Australia, be there April-July. Then accept you’ll be in some regions at the “wrong” time — though this might offer unexpected benefits (for example, Zambia in the wet season means lush landscapes and cheaper prices).
In general, city sightseeing can be done year-round (escape extreme heat/cold/rain in museums and cafés) but outdoor adventures are more reliant on — and enjoyable in — the right weather.