Dear friend,
Thanks a lot for your letter. We have been friends for lots of years now and I am very grateful for your invitation. It means a lot to me. Unfortunately, I have to turn down your invitation and I am very sorry for that. My family and I are visiting our grandmother this Christmas. We planned this trip a long time ago because she lives in another country. I hope to celebrate next Christmas with you.
Best wishes,
[Твоё имя]