Dear Elena,
I'm so sorry! I know that I've been a terrible friend, and I've been feeling awful ever since I realised that I forgot your birthday last week. I couldn't believe it when I saw the date on my calendar.
I'd like to explain how it happened, although I know it's not a good excuse. I've been working extremely long hours on a big project, and I've hardly been home before ten each night. The days all blended together, and I completely lost track of the date. I should have written it in my diary, and I'll do that from now on.
I feel particularly bad because you've always remembered mine, and you even sent flowers to the office last year.
I'd love to make it up to you. How about I take you out for a birthday dinner at that Thai restaurant you love, my treat? We could go this Saturday evening, if you're free. I'll bring your present too, of course.
Forgive me?
Love,
Sasha