All year round is mosquito season in Hawaii. It all depends where your excursions take you. I've never had a mosquito bite when I've been in any of the beach areas. But, step one toe into a jungle area and it's a different story! In my experience of being bit in California, Colorado, Ohio, Illinois and Florida, none of these bites compare to the itchiness of the Hawaiian Flying Pterodactyl Mosquito (they seem pretty big to me)! So if you're going to any areas like that, I would bring repellent.