Running roughly from December to April, the Dry Season is generally thought of as the best time to visit Saigon. March, April and May are the hottest months, with temperatures reaching as high as 39°C (102°F), but there’s less humidity, which makes it much more bearable. In December, temperatures in the early morning can get down to 16°C (61°F), but it’s comfortably warm for a westerner during the day.
If you visit in late December, you’ll get to experience Christmas and New Year in Saigon. Though primarily a Buddhist country, Christmas is celebrated in Vietnam, and you’ll see plenty of decorations up and sales on in the shops.
Note that during Tet - Vietnam’s Lunar New Year celebration, falling between late January and the middle of February - lots of shops and restaurants will be closed, and accommodation will be more expensive. However, the Tet festivities also make it an exciting time to visit Saigon, so if you have a decent budget then it’s still worth traveling at this time.
The Rainy Season sees higher humidity and, of course, more rain. Tropical storms are considerably more frequent, but it’s still a perfectly good time to visit Saigon because the rain isn’t non-stop all day; it usually falls in heavy showers, with sun in between. May to September are a good bet if you’re on a budget, because accommodation prices tend to be cheaper.
It’s worth noting that various public vacations take place over the Rainy Season, and they can mean that restaurants and shops are closed and public transport is harder to come by. The main ones are Vietnam Reunification Day (30 April), May Day (1 May) and Vietnam National Day (2 September).