Best time to visit: The dry season
With a tropical climate of 2 distinctive seasons: dry and wet, it's suggested that you should pay a visit to Ho Chi Minh City during the dry season, starting from December and lasting until April. This period of the year is known to be the hottest, with average temperature of 28 °C (82 °F). The peak is 39 °C (102 °F) around noon time in late April, while the lowest may fall below 16 °C (61 °F) in the early mornings of late December. Humidity is moderate in December and begins to decrease from January to April. Hence, if travelling during this period, foreign travellers can escape from the normal sticky weather of Saigon.
In term of activity, this is surely an attractive occasion as tourists can celebrate Christmas and New Year explosively with local people here (see How Vietnamese celebrate Christmas). From Christmas until New Year, streets and hotels in city center are filled with colorful decorations, people hanging out and grand sales happening at all kinds of shops (including those in upper ranges). During Lunar New Year (around late-January and mid-February), many stores will be closed and air tickets will be fully booked just before it, so plan your trip accordingly. Otherwise it is a great opportunity to see how Vietnamese enjoy their own “Tet” Festival.
The rest of the year
Although dry season tends to be the popular choice, rainy season still has its advantages, considering that this city is endowed with a rare nice climate. It often has short day time rains that clear as quickly as they come, and give space for even more sun.
In this monsoon season, from May to November, it is quite humid and rainy. There're several major national holidays during this time such as Vietnam Reunification Day on April 30th, May Day on May 1st, Vietnam National Day on September 2nd which will see an exodus of people coming in and out of these cities. Transportation and restaurant service on these days may be a little scantier for visitors, especially in nearby retreating places like Nha Trang, Mui Ne and Dalat.
In brief, if you want comfortable climate - visit Vietnam between December and April. If you are a budget travellers or professional sales hunters, May - September is the best choice.