Barra de Navidad sits in the tropical savanna climate zone. The best months to visit are November to April, when the weather is dry and temperatures are warm but not oppressive — daily highs around 28°C, cooler at night. This is also the peak season, so the town is busiest (and prices slightly higher) during December and January when Mexican and foreign visitors both arrive.
The wet season runs from June to October, with September being the wettest month. Rain comes in afternoon downpours rather than all-day drizzle, so mornings are usually fine. The upside of coming in the wet season is that the town is quieter, accommodation is cheaper, and the landscape is dramatically green. The downside is the humidity, which can be heavy, and occasional tropical storms — Hurricane Patricia, the most powerful ever recorded in the eastern Pacific, made landfall not far from this coast in October 2015.
Semana Santa (Easter week) is the busiest week of the year, when Mexican families pack the beach. If you want the town at its most lively, come then; if you want it quiet, avoid it.
The shoulder months of November and April offer the best combination of good weather, manageable crowds, and reasonable prices.