Originally Posted by
mnmag
You can only do a SDC (fee-free for Plat) within 24 hours of the original flight -- so the 'same' flight the day before -- will not be eligible, but any of the flights after that one, would be.
Sometimes the "same" flight operates at different times on different days, so doing SDC to the same flight a day earlier might be possible on rare occasions. I've never tried it myself.