You have missed your friend's birthday party: apologise for missing the party and explain why you couldn't go:-ask your friend how the party was:-suggest meeting your friend to give him or her their present

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)