At the park yesterday, we found the slide.

"Yesterday" is an adverb or an adjective?

Answer: yesterday is an adverb; relation to time

Explanation: adjectives describe nouns. Adverbs answer the questions when, where, how, and to what extent and modify an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree.

It is a noun. "Tomorrow" is considered an adverb!