Puerto Escondido has two distinct seasons that determine what kind of experience you'll have.
November through April (dry season): The best time to visit for most travellers. Skies are clear, humidity is manageable, and the town is at its most comfortable. The surf at Zicatela peaks in November–December, when the international competitions are held and the town fills with surfers. For calm-water swimming and beach activities, January through April offers warm water and reliably good weather.
May through October (rainy season): The Pacific rainy season brings daily downpours, usually in the afternoon and evening. The mornings are often fine, and the rains clear quickly. The landscape around town turns vivid green. Prices are lower, the town is quieter, and the bioluminescence at Laguna de Manialtepec is at its most spectacular (the rainy season concentrates the dinoflagellates). Sea turtle nesting season runs June through December. The trade-off is oppressive humidity in August and September, and occasional rough weather that makes the surf at Zicatela terrifying rather than exciting.
Best for surfers: November and December for the biggest swells at Zicatela. May and June for good surf without the crowds.