Dear (имя друга) I'm so sorry, but I can't go to your birthday party. Because my aunt has got a birthday party too. So I and my family have to go to her Birthday party. Because she's 30 today! But don't worry about me. I hope you will have fun. So don't be sad cause it's your birthday and your day. Anyway I wish you happiness,health,good marks and all that you want! Love you! Your friend(твое имя)