This country only really has 2 different seasons, which is the rainy season and the season where the temperatures are cooler. The rainy season is from June to November, and the rain can be relentless during this time. The best time to visit is the rest of the year, which is from November to May, and during this time the temperatures are cooler, but the air is drier.
If you come during the rainy season, be prepared that some of your plans will get rained out. Trying to visit the area during the rains is probably one of the worst ideas, but it is the cheaper time. However, make sure to determine whether or not you want to brave the rain. Else you can play safe and decide to come during the cooler part of the year so that you can be guaranteed to see everything.
The most popular time to come visit, which is also the easiest in terms of transportation is from October until April.