The peak season in Nagasaki starts from July and continues till August, however the best time to visit Nagasaki is in the months between April and June and again from late October to early November when the temperature a re a comfortable 20-25 degree C.
The winter months from December to February see the average temperatures in the range of 10-3 degrees C. It is cold but not freezing. Tourists are not many and hotel rates low at this time of the year.