Here's what I've found out so far.
Kagami Biraki is held at the beginning of the New Year.
"Kadomatsu" is an arrangement of 3 slant-cut lengths of bamboo tied together with hemp or manila cord and decorated with pine sprigs. The bamboo symbolizes strength and flexibility, while the pine sprigs stand for things everlasting. The cord replaces shinto sacred ropeand symbolizes the bonds that hold aikidoka together in pursuit of O Sensei's goal. All together, the "kadomatsu" stands for morality, virtue and constancy.
"Kagami mochi" are the rice cakes that represent the sun and the moon: there are 2 for increased fortune, and each is rounded to represent smoothness and harmony. The way that mochi is made by pounding the many rice grains that are brought together to make mochi, aikido students train together in the dojo, "pounding" each other on the mat to become unified in comprehension of aikido and eventually "one in harmony with the universe".
"Sake", the rice wine, symbolizes fertility. The god of the new year bears responsibility for the rice crop among other things, and this offering is appropriate for the season. Drinking of sake lifts the spirits to meet the descending gods; it is said that when mortals and gods drink together, they become bonded in celebration.
The "dai dai"(bigarde, or bitter lemon)has been replaced with the tangerine to sybolize new life from seeds. The pronunciation of "dai dai" is identical to the a Japanese expression meaning "generation after generation". The many seeds of the tangerine representing many future generations are like the students of aikido holding potential to start new generations of aikidoka.
"Yam imo" or the mountain yam is a simple tuber that provides tremendous sustenance; therefore it represents the value of simplicity in life, and of being humble and well rooted in the earth.
"Kombu" is seaweed flattened into a sheet. "Moroba" is flat green fern leaf tied in pairs at the stem. What these symbolize I don't know. The image of "the hammer-god" printed on paper represents good fortune for the dojo, as everything this god strikes turn to gold; the paper is traditionally burned after the ceremony to "return to the gods".
Finally "Ebisu" a genial deity of good fortuneis usually shown holding a fishing rod over one shoulder and a big red "tai", or fish, under one arm; the name "tai" is also the last symbol of the word omedetai meaning "festive'or "happy".
This is about all the info I have on this ceremony. I would like to know if any of you have any more information on this.