Kailua-Kona has warm and sunny weather throughout the year.  High temperatures are between 80 and 85 degrees Fahrenheit, and lows are around 70, with very little seasonal fluctuation. 

The rainy season is generally from November through March, while the dry season runs from April through October.  However, there is not a striking difference between the two.   Rain still falls during the dry season, and the rains aren't that much more frequent in the rainy season.

So, whenever you come, you might get showered on, but you'll also have plenty of sunshine to enjoy the beautiful outdoors.

The tourist season is traditionally December through March, as people from the mainland U.S. come to escape their winter climates.  Expect more crowds and higher rates during this period.  However, if you visit in this season, you'll also get to enjoy some extra shops and events that aren't available in the off-season, such as the numerous shaved ice vendors that come out for the crowds.