My response after counting slowly to 10 is "Fine, we'll see you in 6 weeks, it can't be fitted in in between as you're in a schedule".
They're usually happy with that. As a business I personally believe requests to leave it shouldn't be taken personally and that we need to be reasonably flexible, its all part of being professional. Being too rigid and unable to allow for the odd request isn't actually professional at all, its quite silly actually. Especially if you take these requests personally, particularly if they're not meant to be.