Originally Posted by kv99
Can you actually check in 24 hours before the flight at the airport (unless you're on a connection?)
Yes, you can. I've done it a couple of times domestically when arriving one day and departing the next (MRs).
I've used both the counter and EasyCheck-in kiosk. Heck, I've even asked the RCC to give me the return leg of a same-day trip while waiting for the outbound flight.