It seems you misunderstand the meaning of know. It does not describe an action that you perform, but rather a state you are in. You cannot perform the action of knowing something at a specific moment.
You can, however, find out or discover something. From that moment on, you know that something.
You could rephrase your sentence with a present perfect:
We haven't decided where we're going for our holiday yet.
The difference between decide and know is that decide describes an action, so you can talk about it as an action, whereas know is a static verb, which does not describe an action.
Ok, there is actually a case where know can describe an action, the so-called "biblical" sense, i.e. have sexual relations with someone. But a sentence like I haven't known her yet will seem very strange to modern speakers!