I think June is the best season to visit Hokkaido.
The weather here is quite pleasant at this time while most other areas in Japan are in rainy season.
It is still light after 7 p.m, which enables us to enjoy casual evening stroll.
It is also the beginning of fruits season. Strawberries are in season now, and cherry season is coming soon.
I really would like you to visit Hokkaido at this time of year.