On Animal Crossing: New Leaf (and older games) there are many things to do. Some things are digging up holes, mining rocks, shaking trees, going to stores, ect.. When do these things re-appear (restock in store's case)?
The day in Animal Crossing: New Leaf officially rolls over at 6am, not midnight, as one might assume. So if you start playing on Wednesday, the 25th and shake your trees, hit your rocks, buy store items, etc., these things will not reset until 6am on Thursday the 26th.
This also means that you have until this time to finish any tasks your neighbors may have asked you to do for the day, though if they're asleep, you might not be able to turn them in to them.