In South Vietnam, like the rest of tropical Southeast Asia (but not North Vietnam), there are only two seasons: rainy season and dry season. Hanoi has the traditional four seasons with mild but cold winters.

The actual timing of monsoon seasons varies from country to country and there are many regional monsoons throughout the world, including the United States. In Saigon and the Mekong Delta the dry season is from about November to February (around Tet) after which the weather transitions into rainy season around June. April and May can be uncomfortably hot without the respite provided by rains.

The difference is not just in precipitation but also temperature. The temperature range for Ho Chi Minh City throughout the year is generally 25C to 33C, whereas in the hottest days of May it may reach a max of 35C and the coldest nights of December may reach as low as 20C. For travelers who want to enjoy the coolest and driest weather then the months of November, December, and January would be the best, although travelers to Da Lat can come any time of the year and enjoy cool weather.

Tropical Storm Weather

During the wet season, there are also many tropical storms and typhoons off the sea to the east. These storm systems often batter the Philippines and then head to central or northern Vietnam and usually only cause heavy rain to fall on Saigon which rarely gets directly hit.

How should you prepare for Vietnam's weather?

1. Bring an umbrella! Vietnamese people seldom use umbrellas and when they do it's to block the sun not rain. Umbrellas are not that easy to buy in Vietnam.
2. Most of the year it can be pretty sunny although the cooler weather is accompanied by overcast skies. Vietnamese people generally cover their skin with long sleeves and pants (and extra socks, gloves, and face masks) to avoid sun exposure. More foreigners don't, and so may want to use some sun screen which can be purchased at Western-style pharmacies here.
3. You can buy various rain coats here but mostly they are targeting bike riders, who need freedom of movement of their legs thus exposing the legs to rain and puddle splashes. You can buy them for around $1. You can buy cheap and easily ripped ponchos for around 4 per $1 and these come in small enough packs to carry with you. There are some interesting only-in-Vietnam raincoats such as ones with two head-holes so both the driver and passenger on a bike can share one rain coat, and also rain coats with a clear window in front in order to allow a bike's headlights to shine through.
4. Wear comfortable walking sandals. Most tourists walk a lot and they stick out because they're the only ones walking. When it rains your feet will get wet and you may be forced to walk through puddles. You don't want to wear socks or shoes that don't wash and dry easily.

When it rains, it pours

During the normal rainy season, there are many consecutive days where it will rain shortly every day. People who get caught driving in the rain will often pull over and seek shelter in front of shops or at bus stops. They should probably just ride busses more during the rainy season, but that's another topic altogether! It's best not to have any set schedule during the rainy season that way you can wait out any sudden downpours rather than trying to make appointments. You will find that Vietnamese people will use heavy rain as an excuse for not coming somewhere on time.