Celebrations of your friend's birthday are like your own birthday celebrations. When you call someone your friend, it means the differences between you and him/her melt away and you two share a sweet bond of love and affection. Friend's birthday is an occasion for you to express your deep sense of gratitude towards him/her. You want to make this day special and hence plan for grand celebrations. Write a beautiful birthday letter to your friend and I am sure, this gesture will make the day even more special for your friend.
True, these are the days of mails and SMS, and you have to keep with the times you live in; but I would suggest you to send a birthday letter in your own handwriting and send it by post. Your friend will be overwhelmed while reading this letter and preserve that piece of paper all through his life as a token of your friendship. That will be the best birthday-gift for your friend. Even if you two are going to meet for the birthday celebrations, sending a nice letter is indeed a nice idea.
Birthday letter to your friend is an informal way of communication in a true sense. This means there is no specific format to follow. However, the free flow of your feelings should have some cohesion and coherence. Your friend would love reading how you look at his birthday, what you think about your friendship and what is his/her place in your life.
It is not necessary in this case that you have to be a good writer. You may even falter for words. I would say be honest in your manifestation. Let your friend know that you regard him/her as an important person in your life. Here follows one of the drafts of a birthday letter written to friend.
My dear friend Michael,
I hope this letter of mine finds you fit and fine on your birthday tomorrow. Let me wish you a very happy birthday in advance! May God bless you with all happiness, affluence, good health and a long life. I would like you to add life to years when you add years to your life.
Michael, I am proud to have you as my friend. I have always found you with me in all my joys and sorrows in life so far. I have tried to follow your foot steps in all walks of life. You as a student, as a companion and as a human being have always been my ideal. I see this letter as an opportunity for me to express my deep sense of gratitude towards you. Michael after my parents passed away, I was shattered through and through. Then we met and in no time became friends; good friends. You became my support and helped me overcome my feeling of loneliness. You taught me to see beauty in every aspect of life. You taught me to be happy even while suffering from the pangs and pains of life. Your simple living and high thinking always made me and even today make me respect you. I am sure our friendship will ever remain fresh and alive like the bloom of Spring.
I am coming to your place tomorrow in the evening. I am so excited with the thought that we are meeting after almost a month tomorrow. Your birthday party is just a reason. I am all too eager to hear you speaking which always is a pleasure. I am sure you would be reading some of your new poems in the party tomorrow. Let me wish you all luck. I am coming. So long then!
This is just a sample and you are free to express yourself in your own way. There is no fixed format for writing such informal letters. Just make sure your letter brings a sweet smile on the face of the recipient.