On The Sixteen Rules of Esperanto, I found the following description for je.
Every preposition has a definite and permanent meaning, but if we have to use a preposition and the direct meaning doesn't tell us what preposition we should take, then we use the preposition je, which has no independent meaning. Instead of je the accusative without a preposition may be used.
The followed examples are used.
Li vetas je la ĉevaloj. (He bets on the horses.)
Mi alvenos je la oka horo. (I'll arrive at eight o'clock.)
Li vizitos nin je lundo. (He'll visit us on Monday.)
As alternative for the last sentence, Li lundon vizitos nin. is given.
Je is used when you need a preposition, and none of the other prepositions can be used.
There is also the following note.
Since this rule gives us permission to use the -n ending instead of the preposition je, some Esperanto speakers also assume that it permits us to use the preposition je instead of the -n ending. This is a convenience when we encounter a word (such as a proper name) which doesn't lend itself well to taking a regular Esperanto ending.
Mi ja konas Glazunovski-on. (I do know Glazunovski.)
Mi ja konas je Glazunovski.