noun
first three days of the New Year (January 1-3)
Refers specifically to the period from January 1st to 3rd, a time when many businesses and government offices are closed and people visit shrines or relax with family.
三が日は初詣に行く人が多い。
Many people go for their first shrine visit during the first three days of the New Year.
三が日が明けると、仕事始
Once the first three days of the New Year are over, it's back to work.