The whole of Indonesia is tropical with temperatures constantly between 21°C and 33°C whilst at sea level. You can divide the year into wet and dry seasons if you must – but you’ll find that it’s often hard to tell the difference. November to April would be the wet months whilst May through to October are dry. The peak tourist season is between June and September and over the Christmas and New Year period.
If you are concerned over being hit by rain whilst on holiday in Indonesia then you should avoid travelling there between January and February – which is when, strictly speaking, you’ll encounter the most rain. There is one slight anomaly to be aware of which is that in northern Sumatra the regular weather patters are reversed – with wet and dry seasons effectively back-to-front.