15th of August is Obon, unofficial holiday of Japan.
Because it is the Day of the Dead.
In past, most Japanese returned to their homeland on the day
and cleaned their family graveyard,
in order to prepare to be visited their ancestor spirits.
In some regions, people creates vegetable mounts for the spirits
to make their travel from the land of dead easy.
But, this kind of religious importance becomes out-dated now.
Most of us simply return to ancestral family places
and rekindle our kinship by meeting each other.
Some of us consider it as simple summer vacation days,
and go to anywhere suitable for sightseeing.
So foreign travelers should be careful around 15th of August,
as many sightseeing spots' hotels in Japan are often fully booked on those days.