Jinjitsu ( in Japanese) literally means 'Mankind's Day'. It is a custom that was adopted from Chinese culture, which also has its own version of "Jinjitsu" ( ren2 ri4 in Mandarin).
In the Chinese lunar calendar, which was used by the Japanese before the Meiji-era westernisation, this festival was celebrated on the 7th day of the first month. After Japan adopted the solar calendar, Jinjitsu also moved forward - hence it now falls on the 7th day of January, just as the New Year was moved from the first day of the first lunar month to 1st January.