Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* "Yesterday" and "today" specify when something happened or will happen.
Examples:
* Yesterday, I went to the park. ("Yesterday" modifies the verb "went")
* We will meet today at 3 PM. ("Today" modifies the verb "meet")
Let me know if you'd like more examples or have any other questions!