The Yasawas sit in Fiji's western driest zone, which makes them significantly more reliable in terms of sunshine than the eastern islands. The dry season runs from May to October — these are the months when visibility underwater is best, rainfall is minimal, and the trade winds keep things pleasantly cool. July and August are peak season: expect more visitors on the Yasawa Flyer and prices at the higher end of the range.
The wet season (November to April) brings higher temperatures, occasional cyclones, and more rain, but it also brings better dive conditions in some spots (less wind chop on the surface) and noticeably lower prices across the board. Cyclone season peaks between January and March. The Yasawas are not immune — Tropical Cyclone Winston in 2016 caused significant damage across the island group — so check conditions before travelling in those months and make sure your insurance covers delays.
Outside cyclone risk, the Yasawas can be visited year-round. For diving, the visibility tends to peak in the dry season. For budget travel with fewer crowds, the shoulder months of May and October offer a good compromise.