The best time for a trip on Halong Bay is from April to the end of November (the time of summer and fall in Vietnam).
Halong bay is most beautiful in the summer, but it sometimes has heavy rains or even sea storms. In the fall, the weather is cool and ideal for holiday. The temperature is low in the winter, of course not as cold as that one in the West, but it is completely able to cause you a cold. At the time of spring, it’s usually wet and foggy. That is surely not good for visiting the sites and taking photograph.