Day after a holiday automatically means crazy day at work

What is it about the day after a holiday that causes people to be overwhelmed at work? I run a department at a doctors office and I have the pleasure of hearing the office main lines ring every time someone calls in. I understand in order for the doctor to practice and treat patient’s will be calling in to ask the doctor a question and or schedule an appointment. What I don’t understand is why the lines have to be ringing non stop all day from 9am to 5pm. I got in at 9am and the receptionist was on 3 lines a few minutes later all 5 lines were lit up and she was passing calls to the doctor as well.This continued all day long. Yesterday was Labor day and we were open regular hours. Sunday we were open as well. Anytime I had to use the phones I had to watch the lines to see when I could grab one to make some work calls to patients to notify them about changes. So I ask again, why does the day after a holiday cause people to go crazy at a doctors office by keeping all the lines lit up?