The best time to visit Laos is from November to February, the coolest period during the dry season. 
In November, however, a typhoon or a tropical storm may still possible come, and sometimes even in the first half of December. Usually in February the heat begins to rise. The temperature can exceed 35°C. 
The best period will be from middle of December to the end of January, although it can sometimes be very hot even in January. In this period the humidity level is not high and the nights remain cool. It should be noticed that in the winter, it can be cold at night throughout the country, so the minimum temperatures can drop below 10°C; especially in the north where in some cases the temperature can even reach zero degrees. 
In March, April, and May are the season of monsoon, the heat is very strong in the cities and plains, while in the mountains it is more cooler. The weather is still good, although there may be showers and thunderstorms in the afternoon. 
During the rainy season, from May to September, the heat is suffocating, the sky is often cloudy, and sometimes torrential rains can make travel difficult.