Dear Sally,
I am writing this letter to inform and apologize that I cannot come to your party because I have many unexpected things happening beyond my plan.
I have to work overtime because I have a sudden deadline for my job. My company only gives us a few days to finish the given additional work and we must finish the job on time. This is also the reason why I cannot contact you earlier because it is so hectic. Meanwhile, I have just realized that the deadline and your party are at the same time, meaning I have no choice but not to come on your special day.
However, as my apology, if you have free time, I want to meet you to celebrate your special day personally. I think my job will be finished next week and I hope we can meet on Sunday at 10.00 am at Coffe Way.
Hope you will accept this letter and I am really sorry about what happened.
Best regards,
Viviant
