Can I ask why it's your favourite month if the winter specials have left and the summer migrants not yet arrived ? .
Well a lot of stuff is passing through, everything is in summer plumage, the days are getting longer, rarities pop up etc etc. May is one of the best months in the UK too, no?
If time is tight and you want to stay in central Japan the forests at the foot of Fujui are supposed to be good (I've never been) or you could go to one of the Izu Islands by ferry from Tokyo (island endemics, good seawatching and scenic enough for the wife). Again, never been but it is a very popular trip for birders.
I've seen Rubythroats in early May in south Hokkaido (where I live). But they are only passing through..........
Don't know if you use facebook but the kantori group on there is a group for English speaking birders in Japan. You may get some good tips from those folk.