Are days of the week capitalized in Spanish? No. Unlike in English, the Spanish days of the week in are not capitalized.
What is the first day of the week in Spanish? lunes (Monday)
What is the order of the days of the week? lunes, martes, miércoles, jueves, viernes, sábado, domingo
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.