I think yesterday is a noun, because it is definitley not a verb or adjective. It is not an action and it does not describe anything.
Yesterday is a noun in that sentence.
The word yesterday is an adverb.
"Yesterday" is an adverb modifying the noun "afternoon".
Yesterday is an adverb in that sentence.
Walked is a verb.
It's an adverb.
In the sentence "yesterday the leaves hung on the tree," "yesterday" is functioning as an adverb modifying the verb "hung."
"Yesterday" can be an adverb, a noun, or an adjective.ExamplesAdverb: We arrived yesterday.Noun: Yesterday started well. All our yesterdays.Adjective: Yesterday morning
Yesterday he paddled his canoe out into the lake and camped on an island overnight.
She gave a terrific speech yesterday.
part of speech
The part of speech for this particular word is a noun.