[Your letterhead, if desired; if not, your return address]
[Date of letter-month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
I just wanted to drop you a note today to wish you a very happy birthday! Somehow, I did not remember that your birthday was coming up so quickly, and I deeply apologize for missing your special day. I hope you had your most wonderful and fun-filled birthday ever.
As a small expression of my birthday wishes, and as a way to make up for being late with these wishes, I would love to take you out for dinner and a movie sometime soon. Just let me know when would be a good day for you, and we can make some plans.
Again, here's wishing you a very happy birthday, and I'm so sorry that I'm a little behind in getting my wishes to you. See you soon!
[Sender's first and last names]