One of the things I learned on the Minnesota Law Review, and that I later applied while clerking for the Minnesota Court of Appeals, is generally to strike the word upon and replace it with on. Since then, I never questioned the correctness of this substitution because it seemed to make practical sense: Why say upon when on will suffice?
But is this rule well founded and are there any instances in which upon is more appropriate or even preferred?
The authorities are equivocal, but they tend to suggest that upon should be stricken in favor of on in most cases.
Henry Watson Fowler (or at least his editors) says that “[t]he choice [between upon and on] seems almost wholly arbitrary, and there is no saying why one has taken root in some phrases and the other in others.” Modern English Usage 814 (Burchfield 3d. ed. 1996). But in the pocket edition to Modern English Usage published in 1999, editor Robert Allen suggests that “upon tends to sound more formal and emphatic than on when the two are used interchangeably.” (Fowler’s original edition of Modern English Usage published in 1926 gives little guidance either way).
Wilson Follett has a dimmer view of upon: “Everybody uses bookishness as a derogatory term, but many of those who bandy the word about betray the same fault by a habitual use of upon for on, a substitution that in modern prose does as much to produce a bookish effect as any single word can do.” So Follett recommends that upon be reserved for idioms; to avoid ambiguity; and, occasionally, to effect “emphasis, rhythm, or archaic tone.” Modern American Usage 239 (1966).
Bryan Garner generally concurs with Follett: Upon “is a formal word appropriate for formal occasions . . . [b]ut in most contexts upon is unnecessary in place of on . . . . Although some will argue that the two are interchangeable and the choice is just a question of euphony, rarely will upon prove more euphonious or natural. On is the shorter, simpler, and more direct preposition.” Modern American Usage 808 (2003).
Summary: Using upon or on depends on the context. If you want or need to sound more formal (or, as Follett says, “bookish”) use upon. It also is acceptable to use upon when the sense in which it is used is “on the occasion of” or “when (something) occurs.” Garner, Modern American Usage 808. Otherwise, use on in all other cases because it will help you achieve a more conversational tone.
[Aside: Of course, despite these authorities, you could always follow Dr. Grammar, who cites the Merriam-Webster’s Dictionary of English Usage as saying that “On/upon . . . are equally interchangeable.” You can decide.]