Sayulita has two seasons that matter: dry (November through April) and wet (May through October). The dry season is when most visitors come, when the skies stay clear and the Pacific runs warm and relatively calm. December through March is peak season — the town fills up, prices rise, and finding a table at a popular restaurant requires some patience.
November and April are the sweet spots: reliably dry, noticeably cheaper, and without the Christmas-week crowds. If you want surf without company, early November or late April offers uncrowded breaks and cooperative weather.
The wet season isn't without appeal. Rainfall usually comes in short afternoon bursts rather than all-day downpours, and the jungle around town turns a vivid green. The surf is often better — more powerful and more consistent — but the currents can be unpredictable. Fewer visitors means lower prices and a more local pace. The trade-off is humidity, occasional rough seas, and the possibility of a week of persistent rain.
For humpback whale watching, the season runs from November through March, with the peak in January and February. The Marietas Islands are open year-round but access is restricted to licensed tours; book ahead during high season.