Ho Chi Minh City has two distinctive seasons, which is rainy and dry.
The rainy season, also known as the monsoon season, with an average rainfall of about 1800mm annually (about 150 rainy days per year), usually starts in May and ends in late November. During this period, although it will rain heavily almost every afternoon, the rain is short and clear quickly. Sometimes, you can experience sunshine and rain at the same time while the sky is still blue. Remember to bring the raincoat, in case your health is not very familiar with tropical downpour. Especially, in October, there will be extreme tide which may cause severe flood, so be well-prepared with wellington boots and water-proof backpack.
Dry season in Saigon lasts from December to April, is hotter than rainy season (with the temperature around 33 °C) and nearly has no rain. Although travelling in this period is more convenient, travellers need to be careful with heatstroke by avoiding going out at noon.
That leaves the best time to travel to Ho Chi Minh City from December to April, as it is the low domestic season and it does not rain often. The temperate is quite pleasant during this period as well, ranging from 22 to 32 Celcius degree. If you plan to come during this time, be prepared to join Tet festival (Vietnamese Lunar New Year), usually in late January or early February.