This may not be possible in Windows Phone yet. Windows Phone Sand-boxing prevents a third-party app from modifying, altering or launching another third-party app. The only "app" that "should" be capable of doing this is Cortana, but scheduled app launching is not one of her "abilities" at this point in time - probably as Microsoft's back-end developers work more on her this feature might come.
That would be a really cool feature, there. It may not be around in windows phone now, but at the rate that technology is being developed I'm sure it'll arrive sooner than we might expect. All we have to do is live long enough.