The best time to visit Pac Coc, Lang Son Province, Vietnam is during the dry season, which runs from October to April.
During these months, you'll experience pleasant, cooler temperatures and less rainfall. This makes it ideal for exploring the natural beauty of the area, like the various caves and trekking routes, without the discomfort of high humidity or heavy downpours. It's also a great time for outdoor activities such as visiting the local markets and enjoying the scenic landscapes.
While the wet season from May to September sees higher temperatures and more rain, it can still be a good time to visit if you don't mind occasional showers. The landscape is particularly lush and green during this period, and waterfalls are at their most impressive.