Cat Ba Island in October

October through April gives you the best weather. Temperatures hover around 20-25°C and rain is rare. This is peak season though, so book accommodation early. December and January can get surprisingly cool — pack a light jacket for evening. But the clear skies make it perfect for outdoor activities and the limestone karsts look incredible against blue skies. March and April heat up but stay comfortable. This is ideal for rock climbing and hiking before the summer humidity kicks in. Avoid May through September unless you enjoy tropical storms and 90% humidity. The monsoon season turns hiking trails into mud slides and makes rock climbing dangerous. Many tour operators shut down completely in July and August. Vietnamese holidays like Tet (February) and National Day (September) bring domestic crowds. Prices double and beaches get packed. Plan around these dates if possible.