Unlike in English, the days of the week in Spanish are normally not capitalized.
Most of these names end in -es, in which cases the singular and plural forms are one and the same; for example lunes means both “Monday” and “Mondays”. Therefore, to distinguish between “on Monday” and “on Mondays”, it is necessary to use the definite article - el or los.
Tengo muchas citas el martes, pero nada el miércoles.
I have many appointments on Tuesday, but nothing on Wednesday.
Vamos a servicios religiosos los sábados o los domingos.
We go to religious services on Saturdays or on Sundays.