Adverbs are words that modify a verb, an adjective or another adverb. Adverbs of time indicate when the action is being performed. Let's look at some examples:

Yo no sé si yo podré almorzar con ustedes mañana.
I don't know if I will be able to have lunch with you all tomorrow.

Ellos jugaron fútbol ayer.
They played soccer yesterday.

Not only specific days will indicate time, there are other words that indicate when an action is taking place.

Yo voy a buscar su carro ahora.
I'm going to look for your car right now.

¡Jamás le hubiese invitado a salir!
I should have never invited her to go out!

Ella siempre me habla de usted.
She always talks to me about you.

There are other word besides the ones shown in the introduction lesson that indicate time:

cuando

Come los frijoles cuando estén calientes.
Eat the beans when they are hot.

mientras

Rita baila en la discoteca mientras el esposo trabaja hasta la medianoche.
Rita dances at the discotheque while her husband works until midnight.

ya

Los albañiles ya terminaron la pared.
the construction workers already finished the wall.

todavía

Mario todavía te esta esperando en el restaurante.
Mario is still waiting for you at the restaurant.

antes

Mi paquete vendrá antes del próximo viernes.
My package will arrive before next Friday.

después

Tú entras por la puerta principal y después pones las llaves en el llavero.
You go in by the main door, then you place the keys on the key holder.

antaño

Ya no se ven los buenos modales de antaño.
We no longer see the good manners of yesteryear.