A carry-on is when you, and others if you like, go out drinking on say a friday night and you roll in to the next day and even the next night still getting on it, whether it a drug fuelled binge or a alcohol binge, both are classed as a carry-on.
Carry-ons (if your hardcore) can last for several days.
Dave: 'lisshen Steve, It'z 2pm now and weeez gotta meet Leo at the pub at 6pm so we may aswell do a carry-on.'
Steve: 'O shit yeah! Repent and carry-on!'