Since this is the only time many Japanese are permitted to take a vacation, it is the worst time to visit Japan, since every hotel, inn, train, and even plane is booked up months in advance.
Japanese people usually vacation all year long.  
mnli hypothesis: Japanese people usually vacation all year long.   premise: Since this is the only time many Japanese are permitted to take a vacation, it is the worst time to visit Japan, since every hotel, inn, train, and even plane is booked up months in advance.