Erawan Falls are really a series of falls (described as 7 levels). All beautiful all year round. There is more water coming over the falls in rainy season (May-November) of course, but usually more enjoyable to visit in December or January, when there is still good water flow, the trails are drier, and it is cooler. Best to visit early and on weekdays and non-holiday periods to avoid the crowds. You will need more time than you might think to visit as the hike up to the last level takes a good couple of hours -- longer if you want to take a leisurely pace and enjoy the nature.
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