How to reply with regrets

Question:
I've received an invitation to a Bar Mitzvah and the RSVP card is essentially blank - at the top of the card it says "Please respond by the fourth of April". Nothing else - what is proper wording to express our regrets and that we are unable to attend?
Thank you,
Kim
Answer:
You call them tomorrow and tell them that you are terribly sorry, but will be unable to attend.