The difference between the peak and off-peak travel seasons in Japan is severe.
If you have a flexible schedule, try to avoid these particular time periods when prices skyrocket and transportation and hotels tend to be fully booked.
New Year–December 27 to January 4 and adjacent weekends.
Golden Week–April 29 to May 5 and adjacent weekends.
The Summer Bon Holiday–the week centering on August 15.