Hanoi has four distinctive seasons and when you decide to visit depends partly on what your goals are and what you want to do once you get there. So, there really is no bad time and best time to travel Hanoi.
Visit in spring or fall for cooler weather. Summers in Vietnam are very hot, making it difficult to spend long hours outside. In July, temperatures can reach more than 100 degrees Fahrenheit. March and April are good months, with temperatures in the 60s to low 70s. Or you can try visiting between September and November. September is still warm but not unbearable as the summer months; temperatures are in the 80s. It cools down in October, with the highest temperatures reaching into the low 80s. In November, temperatures rarely go over 75 degrees Fahrenheit, with an average low of 66 degrees Fahrenheit.
Travel Hanoi between October to April to avoid the rainy season. Torrential downpours are common during the rainy season, and parts of the city sometimes flood after a storm. Avoiding the rainy season is especially important if you plan on taking day trips out of Hanoi and into the countryside, as roads can be difficult to traverse during heavy rains.
Arrive during Tet, the VIetnamese New Years Celebration, which occurs in late January or early February. Many shop are closed during Tet, so this is not a good time for shopping. However, Tet may be the best time to visit Hanoi because of many colorful parades and temple celebrations. Other major festivals in Hanoi include Co Loa Festival in February, the mid-autumn festival in the fall and the Le Mat Festival in spring. Because festivals are celebrated based in the lunar calendar, dates can vary widely from one year to the next, sometimes with a difference of weeks or sometimes months.