on Wednesday/for Wednesday

[Pooh]

Conversation 1:

"I am scheduled for an appointment Tuesday at 9 o'clock,
and I would like to reschedule."

"We have an opening for Wednesday afternoon.
Can I schedule you for that?"

Conversation 2:

"Let's go fishing together!"

"Sorry, I've got one meeting on Wednesday afternoon.I have to prepare for it."

Could you explain the difference b/w "on wednesday afternoon"
and "for Wednesday afternoon."?
Are they exchangable in this case?

Thanks!! :)