Depends on what season you wish to go, but I would advise for early 2020, before the olympics.
Because in late 2019, the Rugby World Cup will be held in Japan. And knowing what rugby supporters are like, it won't be a good time to visit either.
winter us cold but bearable, it's a good season ti visit, and there's the winter comiket if you're into that.
Spring is probably the best time, weather wise.
Summer is FUCKING HOT. Enjoy being scorched. Only good thing in summer is that you can enjoy both the comiket and wonder festival as the two events are usualy close.
Fall is another good time to visit, but it's usualy quite rainy.