The idea is that Ziggler says he's cashing in at Money In The Bank 2013. I don't know when he says this, probably during the build towards MITB or I could see him saying it actually 2-3 months ahead of time, as a way of building more anticipation towards it. Since last year's event was on July 15th and this year is on July 14th, it's the same Sunday on the calendar so they can play up the whole 'one year in the making" angle but since it's still technically one day separate from each other on the calendar, they can play it up as being the very last day before Ziggler's contract finally runs up. Time is up, and he has to finally make the move to cash in one way or another that night. Ziggler announces at some point that whoever wins their match for the World Heavyweight Championship will have to defend it for a second time immediately afterwards against him, putting extra tension on the winner... Especially since Big E Langston can take the time between the official bell ending the first match and the bell that would signal the beginning of the second match to destroy the champion, allowing Ziggler an even easier cash in.
An extension of the idea (and one that would piss off Ziggler fans for sure) is to have the WHC MITB winner from earlier in the night come out and then cash in on Ziggler and take the title right from him.