Kagami-mochi is a circular, flat rice cake offered to Shinto and Buddhist deities on occasions such as the New Year. A small rice cake is placed on top of a larger rice cake for the offering to deities. There are many variations of kagami-mochi; in some areas, three rice cakes are stacked up, or one of the two layers of rice cakes is colored red in order to represent kohaku (red and white), which is believed to be a good omen (in Ishikawa Prefecture), or shapes are created out of sugar instead of rice cakes, or rice cakes are shaped long and slender and made into a scroll so as to resemble a coiled, white snake.
The name "kagami-mochi" was derived from its shape, which resembled a mirror in the old days. In the old days, mirrors were circular and made of bronze, and were used in Shinto rituals and so forth. It is also said that kagami-mochi is modeled after Yata no Kagami (the eight-span mirror), which is one of the Three Sacred Treasures of the Imperial Family.
Kagami-mochi came to be offered in the style found in the present day after the Muromachi period, when houses began to feature the tokonoma (an alcove in a traditional Japanese room where art or flowers are displayed). In samurai families, weapons and armor were displayed in tokonoma and kagami-mochi was offered in front of the display. It became customary to place yuzuriha (Daphniphyllum macropodum), noshi (a thin strip of dried abalone wrapped in folded red-and-white paper), shrimp, kelp, bitter orange and so forth on kagami-mochi; this style was called Gusoku-mochi (literally, armor rice cakes) or Buke-mochi (literally, samurai family rice cakes). Today, hanshi (calligraphy paper) is placed on sanbo (a small wooden stand on which to place an offering in a Shinto ritual), the umbrella ferns "Gleichenia japonica Spr." are placed on the hanshi, small and large rice cakes are placed on the ferns, and kushigaki (dried persimmons on a skewer), dried cuttlefish, bitter orange, kelp and so forth are placed on the rice cakes.
Today, for the sake of convenience in displaying at home as wel as for good hygiene, many products, such as a plastic case filled with rice cakes to resemble the double layers of kagami-mochi, or a similar case containing many small "individually wrapped" rice cakes, offered in sets with a plastic bitter orange, are sold by rice-cake manufacturers.
The timing at which to start the display
It is said that December 28th is considered the best time to being displaying kagami-mochi. This is because the number eight is considered good due to its shape, which, when written in Japanese kanji, spreads out wide toward the end. In some areas, taian (the most auspicious day in the six-day Buddhist calendar) is selected for the offering of kagami-mochi (December 31st is excluded).
December 29th is considered a day to be avoided because, in Japan, the number nine, which can be read as 'ku' in Japanese, is linked to the Japanese word for suffering (contrastingly, this day is selected in some areas because one way to read the number 29 in Japanese is "fuku," which is also the word for happiness).
December 30th, being a round number, is not considered bad, but selecting this day would be regarded as "one-night rice cake" according to the old lunar calendar, in which the thirtieth is the last day of December.
Displaying kagami-mochi on December 31st is avoided for reasons such as 'lack of sincerity' and 'suggesting the manner of display at a funeral,' and is called 'one-night ornament' or 'one-night rice cake.'
In Jodo Shinshu sect (the True Pure Land Sect of Buddhism), kagami-mochi is offered to sonzen (Shinto and Buddhist deities as well as persons in high positions). Because it's offered as shogon (to decorate Buddha statues and temples) at shusho-e (the New Year's service), kagami-mochi is displayed after the devotional exercises on the morning of December 31st and before the devotional exercises in the afternoon. The offering is based on the principle of not being caught up in the concept of good and bad omens, which is based on annotations for the calendar such as taian and a play on the words for days. Instead of being placed on sanbo, the offering is displayed atop a sheet of white paper placed on a (lacquered) wooden tray. In the hon-sonzen style, the formal manner of display is to use a three-layer kagami-mochi.
The day of kagami-biraki
The day of kagami-biraki (the custom of cutting and eating kagami-mochi) is generally considered January 11th, although this differs from one area to another. Until then, it is considered acceptable to continue displaying kagami-mochi.
After the period for display is over, there is a custom of breaking kagami-mochi into pieces with a wooden hammer or the like and then eating the pieces by making them into shiruko (sweet red-bean soup with pieces of rice cake) and so forth.
Because kagami-mochi has been offered to deities, 'cutting' it with a knife or other implement is considered discourteous and a bad omen.
In Jodo Shinshu sect, kagami-mochi is taken down after the devotional exercises are performed on the morning of January 4th, after shusho-e (for lay believers, during January 1-3), whereupon the rice cakes are made into shiruko, etc., and eaten. No activity suggesting belief in omens, such as those described above, is conducted.