Dear Miha,
Thanks for the invitation to your birthday party on November 20th.
I wish I could, but right now, I am studying and preparing for the midterm exam. This exam will last 5 days from November 18 to 22. Unfortunately, I'm still in the process of taking the exam on your birthday.
It's a pity that I can't attend your birthday party in person. I wish you all the best and the party goes well. I hope that after completing the exam, you and I will have a meeting at a cafe. I have prepared a birthday present, and hope you can arrange a time to meet me.
Thanks for the invitation and please let me know as soon as possible about the time and place to meet if you can.
Love,