(Friend's name), I won't be able to attend your once in a year birthday celebrations. I am extremely sorry. But I promise, I'll come to your next birthday party. Your (age)th birthday will never come again. It's a milestone in your life. Enjoy it. Next time, it's a promise, I will come and be a part of your celebrations. I'm sorry once again. In advance, happy birthday!

I am extremely sorry but I can't come to your birthday celebrations due to some reasons. I hope you don't feel bad because I am feeling bad, how I wanted to come to your birthday and have loads of fun but....So today a this is your birthday don't feel bad. Just Enjoy!! and have fun.. I will miss you but remember your (age) will never come back. This day is special for you. ENJOY THIS LIFE FRIEND!!!!...Happy Birthday!!!...hope you understand :) :)
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