The best months to visit Ba Be are October to April, when the weather is dry and clear. The lake is at its most reflective in October and November, when the air is sharp and the limestone peaks catch the morning light. December and January can be genuinely cold — the park sits at nearly 200 m above sea level and the valley channels the wind — so bring a layer.
The wet season runs from May to September. Rain is heavy and persistent, and trails into the forest become slippery. The upside: the Dau Dang Waterfall is at full power, and the forest is at its greenest. The lake does not flood badly, and boat trips continue through the rainy season. July and August see the highest rainfall.
Ba Be has no major festival that would make it worth timing a visit around. The Tay New Year (late January to February, aligned with the lunar calendar) is celebrated quietly in the villages with communal meals and traditional games — if you happen to be there, you may be invited to join.