The day after a holiday — the first business or workday when normal activities resume.
休日明けの月曜日はメールが多くて忙しい。
Mondays after a holiday are busy with lots of emails.
旅行で疲れていたが、休日明けにはすぐ仕事モードに戻らなくてはならない。
I was tired from the trip, but I had to switch back to work mode immediately on the day after the holiday.
休日明けは会議の予定が集中しやすいので注意してください。
Be aware that meetings tend to be scheduled right after holidays.