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[Tororo Soup on the Third Day of the New Year]

New Year's traditions vary from region to region and from household to household, but in the Tohoku, Kanto, and Chubu regions, there is a custom called "mikka tororo" (grated yam) soup on the second or third day of the New Year.

Mikka tororo is considered auspicious because it is believed to bring good luck due to its association with longevity and good health. Some belief consuming tororo during the period of "Matsu no Uchi" (the time when New Year's pine decorations are displayed) prevents catching a cold. As a tradition to wish for longevity and well-being, people may also apply tororo at the entrance of their homes or scatter it around to ward off evil and ensure that no harm enters.

Yams used in yam soup, such as Japanese yam, nagaimo, and yamatoimo, are nourishing and contain a lot of diastases, a digestive enzyme, so they are effective in promoting digestion. It is also found in Chinese medicine under the name "sanyaku (山薬)," and has long been known as "the eel of the mountains" for its energizing properties.

Mikka tororo can be served over rice or combined with miso soup. Like the seven herbs rice porridge eaten on January 7, yam seems to be a good way to regulate the tired stomach and intestines of the year-end and New Year's holidays.

Ichiyougama's Mortar
https://www.shokunin.com/en/ichiyou/suribachi.html
Azmaya's Wooden Pestle
https://www.shokunin.com/en/azmaya/surikogi.html

References
https://www.maff.go.jp/hokkaido/suishin/shokuiku/osyogatu_gyoji.html
https://ja.wikipedia.org/wiki/ナガイモ