first three days of the New Year (January 1-3)
三が日は店がほとんど休みだ。
Most stores are closed during the first 3 days.
三が日は家族と過ごす予定だ。
I plan to spend the first 3 days with family.
三が日が終わると仕事再開だ。
Work resumes after the first 3 days.