Offdates occur at
Walmart & Kmart. Not at Target. About 1-2 wks before an event is to be run,
IF no one has booked it yet for the originally sceduled date, then more dates will be offered for it. (Ex: If scheduled Sat, off dates usually, Sun - Fri). You would be paid $8 less than the original pay if you do the job on an offdate. So 5 hr event at originally $70, would be paid $62.
If you waited like you mentioned, you risk someone booking the job before you. I book ondate for a store. If events are still available at a 2nd or 3rd store, those are the ones I take as off-dates. Also, you don't want to book off-date at the very last minute if its Walmart. You need time to receive kit in the mail. Kmart & Target kits are delivered to stores.
I suppose if you scheduled on-date, but had a problem arise just days ahead, AND saw the event was indeed going to be offered as off-date (at other stores), you could unbook your store & quickly re-book offdate at your same store.
I did have a recent grand-opening event at Walmart that said it HAD to be run on scheduled date. No offdates were going to be offered even though it was Walmart.