If you both do, effectively, the same job but at different times, then I imagine you both report to the same boss. If so, then you, him and your mutual boss need to get together ASAP to sort this out. He needs to understand that you made a genuine mistake, but that he should have been more self-sufficient and not have relied upon you to provide him with his shift details...
I agree with Mark. You are his equal not his supervisor, this is a genuine mistake. He is an adult and therefore responsible for his own leave records, too - I know when I have a day off, or due back from leave - why didn't he? He can presumably take the "mistake day" back at a later date! Agree that your boss needs to sort it out. "Spark between us" - don't read more into this than you have to. Being mad would be a very odd way to chat you up.