You can visit all the countries in Southeast Asia all year, but there are certain seasons that are better than others.
When To Go: Thailand
North: November to March is the best time of year for trekking in the north. You will still enjoy visiting the north during the rest of the year, but if you are trekking be prepared for rainy conditions.
Bangkok and South: Central Thailand near Bangkok is agreable all year long but can be quite hot in April and May. June through October is the “rainy” or “green” season, but still a great time to visit – you typically expect showers in the late afternoons. The southern islands are less affected by the rainy season.
When To Go: Cambodia
November to March is the best time of year. April and May are the hotest months. June through October is the “rainy” or “green” season, but still a great time to visit.
When To Go: Vietnam
North: Winter in the north lasts from November to April with temperatures ranging from 10 to 16 degrees C (50 to 60 F) in the mountains from January to March, so it can be a bit chilly for trekking with occaisonal fog and drizzle. During the summer months temperatures often reach 30 degrees C (86 F) or hotter.
In the highlands near Sapa night time temperatures can fall as low as 0 C/ 33 F in the winter and rarely go above 26 C/ 79 F in the summer.
Central: Central Vietnam experiences a transitional climate, with heavy rain falls between November and December and dry, hot summer months.
South: Temperatures are fairly constant through the year (25 C to 30 C/ 78 F to 88 F). Seasons are determined by the rains — the dry season runs from November to April and the wet season from May to October. The hottest period is March and April. Typhoons are quite common in coastal areas between July and November.
When To Go: Laos
Laos is a long sliver adjacent to Vietnam, so it has very similar north/ south weather patterns. There is no risk of typhoons since Laos has no coastline.
When To Go: Myanmar
Weather in Myanmar is quite similar to Thailand. Rainy Season: June to September. Cool Season: October to February. Hot Season: March to May. The best time of year to visit Myanmar is from October to mid-April. The coastal areas should be avoided from May through September – due to being situated slightly to the west of Thailand these areas are more heavily affected by the monsoon.