Lanai has a year-round mild climate softened by its position in the middle of the Hawaiian chain, but there is a clear better and worse season. April through October is the dry, warm period — days are sunny, trade winds keep the heat comfortable, and the ocean is at its calmest for snorkeling and swimming at Hulopoe Bay. This is also when the island sees its highest visitor numbers, though even at peak season Lanai feels uncrowded by any Hawaiian standard.
November through March brings more rain, particularly in the highlands around Lanai City, which can be cool and misty for days at a time. The beaches are still accessible and the temperatures remain mild — Lanai rarely drops below the mid-60s Fahrenheit even in winter — but the ocean can be rougher and afternoon rain is more likely. The upside of the wetter months is that the interior is greener and the ironwood forests look their best.
The weeks around Thanksgiving, Christmas, and spring break see a noticeable uptick in visitors, most of them staying at the Four Seasons, which brings some strain on the island's limited restaurant and rental capacity. Those dates are worth avoiding if flexibility allows.
There are no major island-wide festivals that would make a specific date essential. Time a visit around the light: the Garden of the Gods and Shipwreck Beach are most beautiful in winter morning sun, and spinner dolphins at Hulopoe Bay are most reliably present in the early morning hours regardless of season.