Originally, the Chinese characters 七草 meant the seven flowers of autumn; the custom on January 15, the Lunar New Year, is written in Chinese characters as 七種, which are also pronounced as 'na na ku sa,' but generally the custom on January 7 is written as 七草 in Chinese characters.
本来は七草と書いた場合は秋の七草を指し、小正月1月15日のものも七種と書いて「ななくさ」と読むが、一般には7日正月のものが七草と書かれる。