拒絕赴會(飯局) ! (英文題)

One way to reject someone without having to explain or make up a big excuse is -
'Thank you very much for your kind invitation. However, I already have plans for that evening so I won't be able to join you.'
That way it's polite as you have thank the person for their kindness. You've told them you have 'plans' - doesn't matter if it's true or not. And thus, you have explained to them that you can't join them at the table!
That should work =)

I am so sorry that I can't go for dinner on xxth May, 'cos I need to work over-time.
or
I am so sorry that I can't make it tonight, 'cos I don't feel very well.

Sorry! I have already date my friends for dinner!
OR
Sorry, I don't have time for dinner!

不知道!!!!!!!!!!!!!!!!