Re: Have to cancel trip, how to tell my daughter?
Don't write a letter.
First, make SURE that you can reschedule it for after Christmas. Confirm dates so it's not another "wait and see" situation. (My dad did that to me when I was growing up - Said we MIGHT be able to do something, and then always got out of going through with it) So by having a confirmed date, she will know it will happen.
Sit her down and just tell her the truth, and then tell her that your not cancelling the trip, it's just being postponed. She'll be upset, but knowing that she WILL get to see her friend after Christmas will give her something to look forward to. Be careful though, make sure you will follow up with your promise to her.