Please note that if you plan to go to Nan for New Year's Eve, book your accommadation weeks in advance because it is a popular destination for Thais on this occasion. Room prices are also significantly rising at this time.
This festival generally takes place in the month of September
The best time of year to come to Nan is from November to the end of March. August is the rainiest month so maybe avoid this period but during the rest of the monsoon season if the rainfall will often be torrential, they are also brief in general, allowing you to enjoy your stay the rest of the time.
We are one week away from Chinese New Year (January 25th, 2020) and despite a lot of mail and detailed information requests from various Tourist Offices in several cities, I unfortunately did not get all the details that I would have liked...