Being a tropical country, Indonesia is blessed with two seasons, namely dry and rainy. Dry usually occurs from June to September and the rest is rainy season. Sunshine is abundant except in rainy season when the sky tends to be cloudy. It is advisable to visit Indonesia during dry season.
Make sure that your visit does not coincide with holiday festivities such as Muslim holiday Eid (or Lebaran, like Indonesians are fond to say), because the traffic tends to be heavy especially in Java island. Unless you are interested in seeing the festivals up close and experiencing them, of course.
Indonesia’s climate can be hot and humid, so bringing along sunblocks and moisturizers during dry season is recommended. No need to bring umbrellas during rainy season because they are abundant and can easily be bought even in small shops. You might need extra clothing though, and you can purchase them almost anywhere.