Kagamibiraki is an event held to wish for a peaceful year for the family by cutting a kagamimochi (a large, round rice cake) offered to Toshigami (a god of the incoming year) at the New Year, cooking it in zoni (vegetable soup containing rice cakes) or shiruko (sweet red-bean soup with pieces of rice cake), and eating it.
The word 'hiraki' (as in 'kagamibiraki') is used instead of the taboo word 'wari (which means to break something).'
The word 'kagami' means peace, and 'hiraku' (the verb form of 'hiraki') means to increase success.
Eating a kagamimochi is also expressed as 'hagatame (strengthening of the teeth).'
This is to pray to Toshigami for a long life by eating hard food and making the teeth stronger.
In samurai families, mochi or rice cakes offered before the ceremonial display of a suit of armor would be cooked in zoni and eaten, and this event was held to celebrate 'hatsuka (blades and hilts)." This samurai society custom spread to the general public and became the event known as Kagamibiraki. Rice cakes are broken by the hand or a wooden hammer, since a knife would be associated with seppuku (suicide by disembowelment). Also, the event of kagamibiraki carried out by a woman using a kagamimochi offered before a bronze-mirrored dressing table is called a celebration of 'hatsukao (the first face),' and this word is an engo (word associated) with the word 'hatsuka,' meaning the twentieth day. Note that expressions such as 'kiru (cut)' and 'waru (break)' are avoided but instead the lucky word 'hiraku (open)' is used. Originally, Kagamibiraki was held on January 20 (by the lunar calendar) after the Lunar New Year (January 15 (by the lunar calendar)), which was at the end of matsunouchi (the first seven days of the year in which matsu kazari (pine decorations) were displayed). However, because the death of Iemitsu TOKUGAWA occurred on April 20, 1651 (by the lunar calendar), the holding of the kagamibiraki event on January 20 was avoided in the Kanto region in recognition of the anniversary; instead, it was later decided to be held on January 11 (by the lunar calendar) after Matsunouchi. For this reason, although the Gregorian calendar (the solar calendar) is now in use, the event is still held on January 11 after Matsunouchi ends on January 7 in the Kanto region, while in other regions it is held on January 20 (the end of the Lunar New Year). Note that the event is held on January 4 in Kyoto.
The expression 'kagamibiraki' is also used occasionally to refer to the opening of a sake barrel by breaking its lid with a wooden hammer at a feast; however, the correct expression of this event is 'kagaminuki.'