For nearly six hundred years, a river in the town of Tsushima has been the stage for one of Japan's three great river festivals, a ritual now inscribed on UNESCO's list of intangible cultural heritage. It comes in two acts, one at night and one at dawn, and the difference between them is the whole point.

The evening festival, the yoimatsuri, belongs to light. Five boats, each built up with a great semicircular frame, are hung with more than five hundred lanterns apiece. When they are lit and pushed out onto the Tennogawa River, the water beneath them catches the glow, and a second arc of lanterns appears upside down in the black surface. For a while there are two festivals, one floating and one drowned, mirroring each other across the waterline.

By dawn the boats have been transformed. Overnight their lantern frames come down and figures from Noh drama are mounted in their place, and the morning festival, the asamatsuri, sends them out again to the sound of old court music drifting across the river.

The festival began, as so many summer festivals in Japan did, as a way to drive off plague and to ask safe passage through the heat. That original purpose still hums beneath the spectacle. Darkness and light, water and fire, held in careful balance at the very end of July — a prayer that happens to be one of the most beautiful things you will ever see on a river at night.