Let's be honest. No one wants to show up for work when they could be relaxing by the pool. If a coworker is sick or there is an emergency, sometimes you gotta take one for the team. Here's a good response:

"Even though having to work on your day off isn't ideal, I understand that life happens. I'm willing to be flexible and positive and work the shift."

Due to the nature of the job, there may be times you are on-call or have to work on an off day. Show that you have a good perspective and a good attitude.