The best months on Nias are June through October, when the south-west swell is consistent and the weather is generally dry. July to early September is peak surf season at Lagundri and Sorake, when the bay fills with the international surf circuit and competitions are sometimes held.
Outside the surf window the island is much quieter and far cheaper. April to May and November can give pleasant weather and a more authentic feel in the villages — many are happy to host visitors when the rush is over. The wettest months are December to February, with heavy afternoon rain and rough seas; ferries to and from Sumatra are sometimes cancelled.
The Owasa harvest festival (irregular timing, usually after rice harvest) and Christmas celebrations in the largely Christian villages of the south are worth catching if dates line up.