XXXXXXXXXXX (Your address)
4th June 2015 (Date)
Dear XXXXXXXXXX (Your friend's name)
Hope that this letter of mine finds you in the best of your spirits. First of all, I would like to say happy birthday to you, and I am really sorry because I will not be able to attend your birthday party as I have my English test next day. That test is really important and I can't miss it. But please don't spoil your mood and your precious day for that. What if I will not be present there? My love and blessings are always with you, my dear friend. And if you are concerned for your gift, then I'd like to tell you that I will send it to you with my brother because he is coming to your party. So no need to be sad. Once again many many happy returns of the day. Convey my regards to uncle and aunty and to your little sister, lots of love.
Your loving friend
XXXXXXXXXXX (Your name)