There's nothing like an all-inclusive holiday in Chiang Mai to rejuvenate you, especially when you know what to expect from the elements. Here's some information to help you choose when to book: The hottest months are usually April and March, with an average temperature of 28°C, while the coldest months are January and December, with an average of 23°C. February and December are the driest months, while September and August are the rainiest months.