There is no "best" time to visit Honolulu, though summer can be hot and humid. The city's climate is warm and sunny most of the year, though residents do notice a seasonal change, with temperatures dropping to an average of 75-80 F/25-2 C in early November. Things stay that way through May, before the temperature goes soaring to above 90 F/32 C, where they stay until October.
Trade winds blow across the island most of the year, and the air only gets uncomfortably humid when trade winds stop and Kona winds from the south take their place. Kona winds also bring vog—volcanic smog—to Honolulu, thanks to the Big Island's constantly erupting volcano. Vog can cause breathing problems, itchy eyes and uncomfortably dry nasal passages, but it also creates more intense sunset colors.
November-April is considered the island's rainy season, even though the rain showers often come and go in a matter of minutes. The combination of rain and sun makes conditions ideal for stunning rainbows, which can often be seen arching over mountain valleys. Most of Oahu's rain falls in the higher elevations and on the island's windward side, ensuring contoured slopes covered with thick verdant rain forest.
Hawaii also has a hurricane season lasting from late summer through fall, but Honolulu residents rarely experience anything more than somewhat stronger winds and some flooding in low-lying areas. It's not a good idea to go hiking in a rainforest when the National Weather Service posts a hurricane advisory, as flash-floods can wash over hiking trails with no warning.
Every few years, Waikiki is evacuated because of a tsunami warning, but so far, the giant waves have not materialized on Oahu.