Different from Hanoi and the north with a cool winter, Ho Chi Minh city has warm weather year round. Ho Chi Minh city is subject to monsooon climate with two seasons. The wet season lasts from May to October and it rains quite a lot but mostly short downpours.

It get from hot to hotter from March to April.  Air conditioning is standard throughout the city so you needn't worry too much about the heat. 

The biggest national festival in Vietnam occurs in late January or early February and is called Tet.  This is a good time to vist the country, as the streets erupt with color and... well... festivities. It would not be great if you do not like the rush as during Tet, the whole country is in rapid motion. Tet lasts about a week (or, for some Vietnamese party animals, up to a month) and is scheduled around the lunar calendar, so you'll have to check for this year's dates.

The best time to visit Ho Chi Minh City are in dry season, especially from November through March.