Siargao has two seasons that matter for different reasons. The dry season runs from roughly March to May, when the sea is calm, island-hopping is easy, and the island looks its greenest. This is when families and non-surfers tend to visit. The water around the islands is clear and flat, and you can reach the lagoons and outer islands without fighting chop.
The surf season peaks from August through November, when typhoon swells rolling in from the Pacific push up the reef breaks. Cloud 9 is at its best and most powerful during this period. The downside is that typhoons can disrupt travel, and heavy rain can last for days. December through February brings cooler, windier conditions — surfable but not ideal, and the seas can be rough enough to cancel island tours.
The biggest annual event is the Siargao International Surfing Cup at Cloud 9, usually held in September or October. The island fills up during the competition, so book accommodation well in advance if your trip coincides with it.