I spent a month in Langtang valley in April/May 1996. Too much snow. The locals said September was the best month. But they're not going trekking or climbing. I would say early Sept is too early, particularly as the walk-in is low, not like Everest or Mustang. From mid-Sept onwards might be OK, and Oct is considered best. Walking out in early May was humid, muddy and leeches. I assume early Sept would be the same. Only upside is it's such an accessible trek you're only in the mud a couple of days. Once above Langtang village you should be fine, around 3000m. KTM late August is hot, humid and dirty.