January 7 is the day of the Nanakusa or Seven Herb Festival. This day, many Japanese eat rice gruel (kayu) that contains the seven medicinal herbs of spring as a prayer for good health in the coming year (nanakusa-gayu).

In ancient Japan, people customarily gathered herbs in the spring and ate them as an expression of their wish for good health. The herbs could also be presented to a superior as a wish for long life. To make then easier to consume, the seven spring herbs were later added to a gruel.

The custom goes back to the Heian-period, when according to tradition the Emperor Saga was very fond of this broth, although it was in his time eaten on the first Day of the Rat. In the late ninth century, in the days of Emperor Uda, the custom came to be observed on January 7.
The custom of serving the Emperor with a medicinal gruel on January 7 continued till the Tokugawa period, during which the Seven Herbs Festival came to be widely observed in the whole country.

[Nanakusa-gayu - photo Wikipedia]
The seven herbs are:
nazuna, or shepherd's purse (Capsella bursa-pastoris)

hakobera, or chickweed (Stellaria media)

seri, or water dropworth, Japanese parsley (Oenanthe javanica; this is one of the few non-toxic species of the Oenanthe (water dropworts) genus, which are otherwise extremely toxic, so outside of Japan be careful not to consume any wild-growing varieties even in small amounts!)

gogyo or hahakogusa, cottonweed (Gnaphalium affine)

hotokenoza, or henbit (Lamium am...