The day after nobody didn't remember anything.
In this case dopo refers to something, to his birthday, Christmas, the day he died whatever, otherwise the sentence doesn't make sense. That's why dopo in this case is a pronominal adverb. It stands for something and as an adverbial adjunct it describes the time when an event happened. We have to admit that this kind of word is somehow strange because it refers to a noun and adverbs in general don't refer to nouns. But that's like that in all languages (the day after, le jour après, el día después, der Tag danach).
You can see once again that there are problems in this world which are complicated from a theoretical point of view, but no problem at all from a practical point of view. It's enough having heard this construction one time and then you can use it with other nouns, il giorno dopo (the day after), l' anno dopo (the year after), la settimana dopo (the week after) and so on.