The best time to tour around Vietnam by bike is from December to April, when the weather is cool, dry and brilliant. You can drop in late January or early February to enjoy a memorable Tet holiday blooming all over Vietnam though the north may be quite cold this time. In early summer (April-May), temperatures picks up, warming to the 80s or 90s°F during the afternoon, with high humidity. It is not a good idea to cycle around Vietnam during summer and rainy season (May to October).
