Sarawak is equatorial and rainy year-round; "dry" and "wet" seasons are a matter of degree rather than presence or absence. The driest months are June through September, which coincide with the school summer holidays of the northern hemisphere — book accommodation and Mulu flights early. The Rainforest World Music Festival at the Cultural Village runs the last weekend of July.
The wettest stretch is November through February, when the northeast monsoon drives serious downpours and the rivers run high; cave trips can be cancelled with little notice, and Bako boat access becomes unreliable. Avoid these months if you have a tight itinerary. Year-round, expect afternoon thunderstorms, sticky humidity, and full leeches in the lowland forest.