We went there during our 2010-11 year in Sa Kaeo. According to their site, the park is home to an estimated 400 species of butterflies, May to July is the best time for butterfly watching.
Butterfly Watching season at Pang Sida National Park
The rainy season is when our forests are at their freshest and greenest, and it’s also when many different species of butterfly reach the peak of their numbers. So from June until Jul 31, Sa Kaeo attracts eco-tourists to Pang Sida National Park with the promise of “Butterfly Watching at Pang Sida.”
Visitors can take a workshop on how to photograph the winged wonders then enter their works in a most beautiful butterfly contest, peruse a trade fair and partake in a variety of other eco-tourism activities.
It’s also a great time to visit the famous Pang Sida waterfall.
A couple of old threads; unfortunately most pics have gone
Sakaeo Pang Sida National Park
Pang Sida Butterfly Watching Festival 2018 opened in Sa Kaeo
And the Thai National Parks page: Pangsida National Park