There's a quirk to spoken human languages: the shorter the word, the more meanings it has, and the longer a word the fewer meanings it has. For instance, “jack” has dozens of meanings, whereas “pneumonoultramicroscopicsilicovolcanoconiosis” really only has one (or perhaps none, if one assumes the word is never really used in anything but lists of long words). The same is true for Russian. The long preposition накануне has only two meanings, whereas the short preposition по has dozens. Today we will discuss по in the meaning of “on the subject of.” With that meaning по is used with the dative case. First of all, it can be used with nouns that indicate written content, e.g. books and magazines:

The preposition is also used for lots of things associated with academics, e.g. exams, textbooks, handbooks: