Today is the eleventh day after the new year in Japan, and today marks "kagami biraki," or "mirror opening" in direct translation. What it means is that families around the country who have decorated their houses with kagami mochi, or round mochi similar to mirrors that are used in Shinto shrines to symbolize the god Amaterasu, break them apart on this day.
There was an interesting discussion of the event on the radio this morning. It seems that the event was originally celebrated on the eighth day after the new year, but after the son of a Shogun died on the eighth, it was moved to the eleventh. Of course, at that time they used the old solar/lunar calendar rather than the Gregorian calendar that is in popular use today, so it is even different from that day, really.
The word biraki, which actually means open or spread, is used because breaking or dividing carry negative connotations. When the tradition first started, knives were not used, and instead the mochi was broken with a wooden hammer, like the top of a sake cask, an act which is also called "kagami biraki. "
People bake or fry the mochi and eat it with salt or soy sauce. I like it with cheese!