where do the witches plan to meet again, and why

The simple answer is upon the heath.

Aslan is correct; they meet him on the heath. The witches planned the meeting, but for Macbeth, this meeting at least was accidental.

I guess I should add a little. They talk about various equivocations or logical fallacies like "battle lost and won" and then decide that the heath is spooky enough to meets with Macbeth and mess with his mind.