There are two distinct seasons in Saigon: the Northwest Monsoon season from November to April and the Southwest rainy season from May to October.
The best time to visit Ho Chi Minh City is from December to March, the dry season (hot, dry and with low humidity). The temperature is between 20 – 34°C / 68 – 93°F. Many people like to come in January and February to enjoy the Tet festivities. Keep in mind that accommodation prices can rise due to the high demand during this period.
March, April, and May are pretty hot months, but the dryness makes the temperature more bearable. In December, the temperature can drop significantly in the morning (below 20°C / 68°F). This is why it is also a good period to come to comfortably enjoy your trip.
From May to September, tropical rain hits the city. Although you should be prepared for the sudden rain, it is still a good time to go if you are on a budget. This is due to the fact that accommodations tend to be cheaper around this time of the year.