first day back at work after New Year's
Refers specifically to the first working day after the New Year holiday period in Japan, often with a sense of fresh start.
Today was my first day back at work after the New Year's holiday, and I saw my colleagues for the first time in a while.
first task at a new job; newcomer's first assignment
Used when someone starts a new job or position and is given their first task or responsibility.
The new employee's first task was organizing business cards.
Compound of 初 (hatsu, 'first') + 仕事 (shigoto, 'work').