Answer:
Dear Jax
Thank you a bunch for inviting me over for your birthday!
But unfortunately I had been unable to attend it, I am sorry that this notice was not said sooner and I hope you will be able to understand that I had an important meeting at the time the party had started. I will have to take you for a birthday bash when I am free again, it will of course be my treat. You can think of this as being a second birthday minus the addition you being another year older- who would want to age twice within a span of a week?
For the next "Birthday bash" I was hoping to take you out to your favourite restaurant where you will be able to enjoy your by far all-time favourite dinner! With of course a dessert of your choice.
Just let me know when you are next free and I will sort the event out!
Best wishes and a happy belated birthday.
Simon.