river-opening festival; start of boating season
A traditional summer event held at rivers, often featuring fireworks and boat rides, marking the beginning of the boating and river recreation season.
At the Sumida River opening festival, a fireworks display is held every year.
On the day of the river-opening festival, many pleasure boats float on the river.
Compound of 川 (kawa, 'river') and 開き (biraki, 'opening'), referring to the ceremonial opening of the river for the season.