What exactly is meant by "these can only be used on their own" when talking about "le jour d'avant/d'après"? Maybe I'm missing something obvious but for me the examples don't really encapsulate the concept. Do you mean that they can't be used to detail more about the thing that happened - only that which happened before and after it? For example you couldn't say:"le jour d'avant de la visite de ma tante" (the aunts visit being the springboard for what has happened the day before or after)
but you could say it when using "la veille":
"la veille de la visite de ma tante"
(and vice versa for le jour d'après and le lendemain)?Looking at the examples above of:"Le jeudi d'après, elle était partie.""Le mercredi d'avant, elle lui avait dit toute la vérité."etc.,
this is the only way I can see that le jour d'avant/d'après are different. They talk about what happened before/after the thing, but nothing more about the thing itself.
Please let me know if I've got this completely wrong... :-)