Scatty, I'm a step-daughter.
He cannot be allowed to mess with arrangements from this point forward.
As of now, his days with his daughter are fixed in stone and if he doesn't like it, you'll be saying "No" when he starts faffing around, offering alternative dates - end of.
It's vital that you take control of this now. At the moment, he and his wife are holding all the cards and pulling all the strings. He's jerking you and your daughter around.
His wife MUST fall into line as well. For your daughter's sake, continuity and thus, her feelings of security within her relationship with her father, are paramount.
Please listen to me - it doesn't happen offten on here but on this subject, I do know my stuff.
If your DD is secure in her relationship with her Dad, she'll handle the step-mother rubbish with confidence. As it is at the moment, her confidence is naturally low and being beaten further in to the ground because her Dad appears not to have any regard or respect for her special time with her. That's the crux to this one.