Kagami-biraki refers to the New Year’s tradition of dividing up offered mochi and eating it.

The Edo Shogunate called this event Gusokuiwai (celebration of armors) and offered rice cakes called gusoku mochi to a full suit of armor (gusoku). Every year on January 11, the alcove of the Kuro Shoin drawing room would be decorated with the "Fern Armor" worn by Tokugawa Ieyasu, the founder and the first Shōgun of the Edo Shogunate, swords, and other battle regalia, to which the gusoku mochi would be then offered. Since Ieyasu wore this armor in victorious battle, it was an extremely auspicious item for the Tokugawa household.After eating his celebratory meal, the Shōgun would grant an audience to the Fudai Daimyō (those who served the Tokugawa household before it seized power) and government officials then give them sake and mochi.