The ideal time to visit Chiang Mai is between October and April. Weather during this period is mostly cool and pleasant with light breeze, which is also why it’s peak tourist season. Another good time to visit Chiang Mai is during the festivals when the city is at its vibrant best. Here's a monthly break up of season and events so that you can plan your trip better?
November to February: This is the ideal time to visit Chiang Mai because the weather is perfect for all kinds of outdoor activities. While it does not get too cold, you should carry a jacket and some light woolens. January is usually considered the best month and sees a high influx of tourists. If you plan a trip in November you will catch the Loi Krathong festivities and in February you will see the Chiang Mai flower festival.
March to May: While summers start setting in from the end of March, tourists still visit Chiang Mai and it's a good time to look for deals on hotels and flights. Even if the temperature during the day is high you can spend the afternoon in the pool and head out after sunset. Temperatures rise up to 40°C between April and May. Carry light cottons, sunglasses and hats to beat the heat.
June to October: The temperature during the rainy season cools down although the humidity rises. While it rains less than southern Thailand, monsoons are still heavy in Chiang Mai as well. So you will have to be prepared with umbrellas, raincoats and study shoes. Not many tourists visit Chiang Mai during the monsoon so if you are looking for a quieter holiday in this ancient town, then this is the time for you.