We insist upon you reading this informative article by Tim Bowen!
The preposition upon is generally much more formal than on but can be used to replace it in certain situations.
It can be used instead of on after several common verbs, such as happen, depend, insist and congratulate. Examples are ‘He insisted upon seeing you even though I told him you were busy’, ‘A police patrol happened upon the robbers as they were running out of the bank’ and ‘My whole future depended upon the decision of one manager’.
Upon is also used to mean ‘immediately after’, as in ‘Upon his release from prison, Davis went immediately to his mother’s house’. It can also mean ‘happening soon’ and in this case it is not normally replaceable with on, as in ‘Christmas is almost upon us again’ and ‘It’s June already and the exams will soon be upon us’.
When used between two nouns that are the same, upon emphasizes the large number or amount of the thing that has been mentioned, as in ‘I’ve written to you year upon year but I have never received a reply’ and ‘They drove for days across mile upon mile of open desert’.
Upon is also used in a small number of phrasal verbs, notably set, put and chance. Set upon is often passive and means ‘attack’, as in ‘He was walking through the park when he was set upon by a gang of youths’. If you feel put upon, you feel exploited because you are doing all the work while others relax. If you chance upon (or chance on) someone or something you come across them or it unexpectedly, as in ‘I was browsing in a second-hand bookshop when I chanced upon a rare first edition’.