apologise for missing the birthday celebration
explain why you missed it and why you didn't tell your friend
and say what you would like to do to show that you are sorry.
范文：
Dear Mark,
I hope you're well and that you had a fantastic time on your birthday.
I'm so sorry that I wasn't there to celebrate with you, and for not letting you know beforehand that I wouldn't be able to come. I feel terrible that I haven't found the time to contact you until now.
The truth is that I've been under a lot of stress at work recently. I was in danger of losing my job if I didn't meet a sales target, so I haven't been able to think about anything else over the last few weeks. This is why I couldn't be at your party, and why I didn't call to tell you I wouldn't be there.
I really hope you'll forgive me for being so thoughtless, and I hope you'll let me make it up to you. By way of an apology, I've bought tickets for us both to go to the football next Saturday; I hope you'll come.
Speak to you soon,
Mike(168)
