Dear Sir,
This is to inform you that I got an interview notice for the position of HR Manager at your prestigious organization, yesterday via email. My interview date has been decided on 7th January, 2015. Unfortunately, I am not available on that day as I have appointment with doctor and I have to take my mother for her monthly checkup.
I extremely apologize for asking the cancellation and rescheduling of my job interview. I request you to please reschedule my interview as I am available after the specific day. I shall be looking for your prompt attention in this regard.