The best time to visit Koh Si Chang is from December to February.
Thailand broadly experiences three types of seasons during the year; summer, winter, and the monsoon. The months of December-February is particularly conducive to travel because it lies in the middle of the cool season and also promises uninterrupted travel. Travelling to Thailand during May is generally avoided because of the thick tropical summer that envelopes most parts of the nation.
Come the winter season, however, the weather becomes far more agreeable. The rainy season threatens logistics of a trip as transportation becomes difficult and other difficulties may arise as a result of the poor weather conditions. The various tourist hubs and the famous palace will be more difficult to access during the monsoon and the soaring temperatures of the summer will not be helpful for travel.