The sake barrel kagami-biraki is said to trace its origins to another kind of kagami-biraki involving mochi, or “soft round rice cakes” which is performed to commemorate the first working day of the New Year or the commencement of events. In the past, samurai households would at New Years make an offering to the gods of a stack of mochi to represent the kagami. The mochi were cut up into pieces to represent the biraki, or “opening,” and eaten on January 11. Even today, most households and offices observe this custom, placing kagami-mochi on their kamidana (a small Shinto altar usually set on a shelf over a lintel) at New Years.
Both types of kagami-biraki - the breaking open of a barrel of sake at Shinto blessings and celebrations, and the cutting up of kagami-mochi - are a means of beseeching the gods to grant good health and fortune at junctures in time, such as the New Year or the start of a new departure in life or business. For such reasons, kagami-biraki is becoming an increasingly popular way of launching house-warming parties, company foundation day events, wedding receptions and other celebrations.