“لِ” (li)means “for / to.” It is a preposition: it links this word to the next word or phrase.Corpus grammarP — prefixed preposition.
“يَوْمٍ” (yawmin)means “For a Day” in this verse. It is a noun. a naming word. Here it is masculine, not specific, genitive.Corpus grammarN — genitive indefinite noun.