Monday would be OK, but I may have to cancel an appointment. Er, Tuesday or Wednesday mornings are possible.

星期一可以，但我可能得取消一项预约。嗯，星期二或星期三上午可以。

Now, this is a good expression to cancel an appointment with someone that you don't know and most English native speakers will understand this expression.

这是一个很好的用来和不熟悉的人取消预约的表达，大多数英语母语者都会理解这个表达。

But let's be real, sometimes we aren't as organised as we want to be and we need to change or cancel an appointment without much warning, right?

但让我们现实一点，有时我们没有我们想要的那样井井有条，我们需要改变或者是取消计划，但是却没有什么提前警示，对吧？

I'm going to assume that this is the correct sentence because I want to know why he cancelled an appointment when he has a patient coming.

我假设这是正确的句子，因为我想知道他为什么在有病人来时取消预约。