Hi, as you didn't mention that on which occasion you want to send a invitation, so I took the invitation for the marriage. Here are both letters for accepting the invitation and for expressing your excuse, giving the reason for not to accepting the invitation.
Dear Alex,
You will be very glad to know that the marriage of my elder brother falls on 25th December,2015. I cordially invite you to attend the marriage and grace the occasion.The marriage party will leave for Agra on 25th December early in the morning. I have invited all my friends. We shall have a jolly good time together. Uncle will present his dance and Rajesh will present a melodious tune on the orchestra. It will be a nice occasion to enjoy.
I hope you will not disappoint me.
Yours Sincerely,
Deepak
My dear Deepak
I have no words to thank you for inviting me on the auspicious occasion of your brother's marriage. I am unlucky to grace this occasion because of pre-medical test already fixed for 20th December,2015. You know I can not miss this chance that luck has provided me to avail. So it is quite impossible for me to miss my chance. I, therefore, regret my inbility to take part in the celebrations of your brother's marriage.
I believe you will excuse me.
May God shower all His blessings on you & your brother!
Heartiest congratulations.
Yours Sincerely
Alex.
