Right to refusal child custody question?

If my spouse who is non-custodial parent cannot take the children for the first three days of his christmas time with them and offers me his right to refusal and I accept am I required to make that time up with my time? I have had things planned in advance and cannot help it he cant afford to take off work. He could if he wanted to. What is required? I cant find it in the guidelines that it requires me to make it up to the ex-spouse.