Explanation:

There is a lot of work in hand. Let's cancel the picnic.
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. ... Upon is also used in a small number of phrasal verbs, notably set, put and chance.
Use "the" with any noun when the meaning is specific; for example, when the noun names the only one (or one) of a kind.
“A” is used before words starting in consonant sounds and “an” is used before words starting with vowel sounds. It doesn't matter if the word is an adjective, a noun, an adverb, or anything else; the rule is exactly the same.
"On" is used to indicate position, usually indicating that something is on top of something else. We might say, "My journal is on the desk." In this function, "on" typically denotes proximity or position. Another example would be, "He sat on the stone wall."