The thanking letter is written and explained below,
To,
The Head of XYZ Firm.
Dear Sir,
I am glad that you have invited me to the joyous event of your 25th wedding anniversary. I would like to provide my heartiest congratulation to you and your wife on this auspicious event and may God provide you and your wife health and more joyful years together.
I am sorry because I will not be able to attend the event. I am currently on my annual leaves and out of country on summer vacations at Maldives with my family.
Congratulations again to you and your wife.
Regards,
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