Jakarta basically has a tropical monsoon climate and despite its close proximity to the equator has a very distinct wet and dry season. The weather is hot and humid irrelevant of the month and the temperature is never too cold, even in December and January.
Most people consider the dry months of June – September the best time to visit Jakarta. This is because one can easily tour most of the city without worrying about being drenched all the time. But Jakarta during the wet season also is quite pleasant once you get used to the frequent rains. Even during the rainy season there are frequent days with sunny days or just a light shower.