Sinharaja is a World Biosphere Reserve since 1978 and UNESCO World Heritage Site since 1988. Parts of the primary forest were saved from commercial logging by its inaccessibility.
It frequently rains here at Sinharaja. The best months with less precipitation are January to March and August, September. Anyhow, if you visit Sri Lanka, it's a must to come here with a minimum stay of one night rather two. Sinharaja is a biodiversity hotspot with endemic trees, wildlife, reptiles, amphibians, and birds. One of the rare places with a stunning primary tropical rainforest.
The Sinharaja rainforest is affected by the southwest monsoon with heavy rainfall from May to July and the northeast monsoon from November to January. Driest months and the best time to travel from January/February until March. April, and August to October expect more precipitation but still worth a try. Located at the island’s wet zone, rain can always occur also in the drier season. Where there is rainforest there is rain. The annual rainfall ranges from 3000 up to 6000 mm. Night temps are above 20°C / 68° F and day temps are almost 30° C / 86° F with an average humidity of 85%.
Needless to say to watch birds you need to be there very early. To spot a frogmouth sleeping ask your guide. Usually, they know where to find this rare to spot nightjars.