1. Thank you very much for your invitation to spend three weeks next August with you and your family in Scotland.
2. I would love to come, but unfortunately I am/ will be unable to accept it.
3. My cousins, who live in Canada, are staying with us from August 3rd to August 18th.
4. As it will be their first visit to this country, I am planning to show them around.
5. Is it possible to visit you in September instead?
6. I will certairlly need a rest after my cousins go back to Canada
7. Please tell me if September is convenient for you.
8. I am looking forward to seeing you all again.