The dry season (May through October) is the best time to visit: lower humidity, cooler temperatures, less chance of being rained out during the volcano hike, and clearer skies for views from the crater rim. July and August are the most popular months and accommodation books out early.
The wet season (November through April) brings high humidity and heavy rain, which makes the unpaved roads to the volcano genuinely difficult and can close access entirely. Cyclone risk peaks between January and March — Vanuatu is one of the most cyclone-prone places on earth. That said, the volcano erupts regardless of season, and visiting outside peak season means fewer people on the rim.