resuming work after the New Year's vacation; first work of the year
Refers to the first day back at work after the New Year holiday, or the first time one works in the new year. Often used in business contexts.
今日は仕事始めで、久しぶりに会社に行った。
Today was the first day back at work after the New Year's vacation, and I went to the office for the first time in a while.
仕事始めの日は、みんなで新年の挨拶を交わす。
On the first workday of the year, everyone exchanges New Year's greetings.
Many companies have their first business day of the year on January 4th.
All employees exchanged New Year's greetings on the first day back at work.