Write a Reply to the Above Letter Refusing the invitation

Dear John

I am very happy to receive your letter inviting me to attend your birthday party. However, I regret to inform you that I am not in a position to attend your party.

In fact, my mother has been ailing seriously for the last fortnight and I have to devote my entire time in looking after her. Had Mother been all right, I would certainly not have missed such an auspicious occasion.

I hope you will excuse me for refusing the invitation. Accept my best wishes for your birthday. May God give you a long, happy and prosperous life.

Yours sincerely