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 Kona area has a unique microclimate for Hawaii, because of its position in the lee of three major volcanic mountains (Hualalai, Mauna Kea and Mauna Loa). While for most of Hawaii, the winter months are generally the rainiest, Kona's drier season is generally from November through March, while the rainy season runs from April through October. However, there is not a striking difference between the two, especially in the area around Kailua-Kona.

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 continental North America 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 attractions that aren't available in the off-season, such as the numerous shave ice vendors that come out for the crowds.