2 new sections will be added to RPR and both are going to be available only to registered members that have reach the "The All-Seeing Eye" level that require 750 posts, beware that moderators will ignore abuse by posting meaningless subjects to get the 750 posts. https://redpillrepublic.online/showthread.php?tid=175
For me, I don't mind hot and humid. I'm not a fan of winter though, and Japan housing has very poor insulation. In short, I don't want to be in Japan during any month when it is still cold. (An exception might be Sapporo which has well-insulated homes, I heard).
So for me, April/May/June, perhaps? Although I'm technically fine for any summer month. I do like that the beach culture starts June 1st, I believe. No one goes to the beach until the specific day, which I think is June 1st. So I'd want to be in Japan for that.