For much of the month of September, after a disappointing summer, Tokyo basked in temperatures of 25C or more. In most countries there would be a rush for the beach, but not here. Overnight on 1 September, beach-going Japanese become as rare as buttered sushi.
It is a spectacle as enigmatic as any annual mass migration in the animal kingdom.
Visit the sun-blessed sands near Tokyo any day in August and you'll witness millions ambling out from their air-conditioned roosts into the soupy afternoon heat and heading for the waves.
But once August is over, nothing. Emptiness.
This year the end of August was unseasonably cold, so you might have expected the return of heat and sun for the rump of September to be greeted with whoops and splashes. But no, the shore remains painfully jilted even at weekends. Only dogs, their owners, and a few gaijin (foreigners) take to the forsaken coastline now the party has ended.
And it's not a sea-dousing that the Japanese deny themselves. Outdoor pools lock their doors too, even if the temperatures are in the high twenties.
When I questioned the local authorities in charge of Tokyo's best sea resort, Isshiki Beach at Hayama, ranked by some in the world's top 100 a spokeswoman told me it was closed because "it's not hot and it's not summer" - even though the thermometer that day was registering 28C and everyone around me was sweating in the sun.
To someone like me, who has growing up cursing the leaden clouds of northern Europe, and with skin a shade of blue for the want of sun, this seems a criminal waste.
Surely the calendar alone cannot be a good reason to desert the briny playground in this city on the same latitude as Malta?
My friend, Nobuo Sato, still dressed in his shirt sleeves, tries to explain it to me.
"Basically, Japanese people are very law-abiding. We have been taught not to swim when there are no lifeguards," he says. "Same with traffic lights. You don't see many people jaywalk here do you?"
It's true. Even along car-free country lanes, people tend to wait patiently for the cross signal.
What happens on 1 September is that lifeguards vanish, because local authorities pull them from the beaches and close all amenities. Summer-season beach bars and restaurants are also meekly dismantled.
"Many of us are so submissive to authority that we will never think to challenge the status quo," says Sato.
Indoctrination starts at school. Children are drilled: "Follow the rules. Don't be selfish. The nail that sticks out gets hammered down."
They learn that custom decreed long ago that autumn (the time you halt beach trips and instead do autumny things) returns at midnight on 31 August precisely. Only a barbarian would be foolish enough to disregard "correct behaviour" that has been established over generations by broad consent.
"Most Japanese are very conscious of the 'four seasons' and what they should do for each season," another Tokyoite and beach lover, Yukiko Oono, tells me.
Autumn in Japan
Kyoto and its royal courts were once strictly regulated by the changing seasons - many of the ancient traditions still exist.
- Shokuyoku no aki (time of hearty appetites) so as the heat dies down, the Japanese enjoy culinary treats such as maple leaves in tempura
- Tsukimi (moon viewing) when people stand on a hill with lashings of tea to view the harvest moon which is thought to be larger and more radiant than at any other time.
- Dokusho no aki (autumn reading) because the shorter days make one more reflective than during the brassier days of summer
- Supotsu no aki (autumn sport) as students enjoy the "crisp autumn air", despite the fact that typhoon season makes early autumn here anything but crisp
It's often been said that Japan operates via strict social norms to achieve its remarkable social cohesion and that this is largely responsible for making Japan such a pleasant place to live. There are no bins at the beaches, nor on the streets. By and large, Japanese people take their rubbish home with them. Just one example of the positive benefits of this cohesion.
Meanwhile, the Japanese feel strongly the judgemental gaze of their fellow citizens. "Such social pressures make the Japanese seem exceedingly conformist in Western eyes. They seem to do everything en masse," says Prof Timothy Takemoto, creator of the Japanese Culture blog.
"One explanation formulated is the 'kata factor'. This argues that based on the traditional arts, such as karate or the tea ceremony, the Japanese learn a variety of 'kata' - schema, or appropriate forms of behaviour for a variety of situations - which they apply in everyday life," he says.
Not going to beaches after 1 September or changing to short sleeves on 1 April may be an example of a "kata".
I, on the other hand, made it a priority to visit the beach in September as a solemn expression of my inalienable rights as a gaijin. This, despite the other reason Japanese say they don't swim in the ocean after 1 September - jellyfish.
"There is a firm belief [in Japan] that the jellyfish will come and get you after Obon," explains Amy Chavez, who leads an enviable beach-house life on an island in the west of Japan, referring to an ancestor worship holiday in mid-August.
"Ah yes, there is a superstition element in this," agrees my publisher friend, Nobuo Sato. "My parents didn't want us kids to go to the beach after the Obon period. They warned us that the spirits of our ancestor will drown us. We went to the beach anyway, but we always had some qualms about this."
Maybe I should have paid more heed to this warning. The last time I attempted an ocean swim in Hayama I was immediately stung by a jellyfish. The ancestors evidently were not pleased.
Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.