Buy it at least a day ahead of time. On the last day there is a small late fee. It is better to enter online. It will save you some time waiting in line. Everyone who wants to play gets in. I've never seen them limit the field.
So the last day you can buy 9-ball entry is on the 23rd? What about after the DCC starts, but before the 9-ball event starts (as in, could you buy in on the 25th or something)?