noun
breaking open a ceremonial sake barrel
Traditional Japanese ceremony where the lid of a sake barrel (called 鏡) is broken open with wooden mallets, often at weddings, parties, or celebrations.
結婚式で鏡割りが行われた。
The kagami-wari ceremony was performed at the wedding.
新年会の鏡割りで、参加者に振る舞い酒が配られた。
At the New Year's party kagami-wari, sake was served to the attendees.
Compound of 鏡 (かがみ, 'mirror', here referring to the round lid of a sake barrel) and 割り (わり, 'breaking'). The lid is called 鏡 because of its resemblance to a traditional round mirror.