Yakushima during end of March - early April

I was planning on doing an overnight over the mountain, past the Jomon Sugi, and down through the Mononoke Forest, with a large-ish group. There are some that haven't been on an overnight hike before so I was just wondering what the weather conditions are like around that time of year. i.e. How wet is it? What is the temperature like? And what is the difficulty? I'm aware that it will be at least moderately difficult but any information would be great for helping me appropriately prepare myself and the others for the hike.

You can hike from mononoke to jomonsugi and back in one day (about 12 hours in total) if you want. End of march / early april should be good weather for hiking. The temperature will be comfortable at at sea level but maybe a bit on the cool side once you get to jomonsugi. If you do spend the night up there, you'll definitely want some warm sleeping bags / clothes etc. Although it doesn't rain as often in yakushima as they claim, it's still best to be prepared for a downpour at any moment.

The difficulty depends on how good of shape you're in to begin with. My first time was really hard for me (I'm young and not overweight but didn't exercise much, never went hiking), and I did spend the night at the top...I'm not sure if I could have done it in 1 day. But 2nd time, 3rd time etc. I could easily do it (mononoke to jomonsugi) roundtrip in 11 or 12 hours roundtrip. And 80 year old japanese women were passing me on the trail! Parts of it are really easy but it does get very steep towards the end. Hope that helps.