Kagami biraki

lit. 'opening the mirror'; understood as "breaking of the mochi" (鏡開き) is a traditional Japanese ceremony where are broken open. It traditionally falls on January 11 (odd numbers are associated with being good luck in Japan). The term also refers to the opening of a cask of sake at a party or ceremony.

History
The fourth Tokugawa, Tokugawa Ietsuna, was the first to hold a ceremony 300 years ago. On the eve of war, he gathered his in his castle to break open a sake cask. Upon achieving victory, a new tradition was born.

Ceremony
The ceremony nowadays is also performed at weddings, sporting events, housewarmings, opening days at new companies, and other significant events worthy of being celebrated.

In Japan, mochi was traditionally made at home, but most families today buy it ready-made. Over the holidays, a pair of round mochi the size of small plates – one a little larger than the other – is stacked on a stand and placed in a household Shinto or Buddhist altar or  as an offering to the deities that visit on New Year's.

The ornamental mochi is removed on January 11 and broken into smaller pieces before being eaten. By this time, the is usually quite brittle, and cracks appear on the surface. The mochi is not cut with a knife, since cutting has negative connotations (cutting off ties) and is instead broken with one's hands or a hammer.

Many Japanese martial arts dojo use the ceremony to signify their first practice of the New Year.

The sake version of the ceremony (based on the original practice) involves presenting a wooden barrel of ("celebration sake") to the celebrants at the beginning of the event. The round, wooden lid of this barrel (representing harmony) is then broken open with wooden mallets handled by VIPs (thus the event breaking open good fortune) and the contents then freely distributed among the participants.