When used with the days of the week, the definite article has the special meaning "on."

When used with the days of the week, the definite article has the special meaning "on."

No trabajo el lunes.
I don't work on Monday.

No trabajo los martes.
I don't work on Tuesdays.

Hay una fiesta el miércoles.
There is a party on Wednesday.

Hay muchas fiestas los viernes.
There are many parties on Fridays.