Kakadu has two distinct seasons: Wet and Dry. The Dry (May-Oct) is overwhelmingly the best time to go, with temperatures around 86°F (30°C) and sunny days. Many tours, hotels, and even campsites are booked a year in advance, so make sure you have reservations.
In the Wet season, November through April, floodwaters cover much of the park, some attractions are cut off, and the heat and humidity are extreme. Some tour companies do not operate during the Wet, and ranger talks, walks, and slide shows are not offered. The upside is that the crowds vanish, the brownish vegetation bursts into green, waterfalls swell from a trickle to a roar, and lightning storms are spectacular, especially in the hot "buildup" to the season in October and November. The landscape can change dramatically from one day to the next as floodwaters rise and fall, so be prepared for surprises -- nice ones (such as giant flocks of geese) and unwelcome ones (such as blocked roads). Although it can pour down all day, it's more common for the rain to fall in late-afternoon storms and at night. Take it easy in the humidity, and don't even think about camping in this heat -- stay in air-conditioned accommodations.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.