Dear Zhang,
First of all,I give you my late blessing to your birthday.
I am awfully sorry for my absence from your birthday dinner yesterday evening.It was because I had an old friend who comes from Beijing to meet yesterday.He was a stranger here and I have to pick him up from the airport.When I came back home,it was 9 o'clock already and unfortunately,it was too late for me to attend your birthday dinner.And I do apologize.
I am wondering whether you will forgive me or not and that's why I write the letter.I am looking forward to your prompt reply.
Best regards
Yours,
XXX