first business day of the new year; reopening of offices after New Year's holiday
Refers to the ceremonial resumption of work at government offices and many companies, typically on January 4th. Often contrasted with 御用納め (the last business day of the year).
See also: 御用納め
For government offices, January 4th is often the first business day of the new year.
On the first business day of the new year, New Year's greetings are exchanged in each department.