The days of the week in Spanish are not capitalized and are all masculine.Calendars begin the week with Monday, not Sunday.
The definite article is not used after es when telling what day of the week it is.
Hoy es jueves. Today is Thursday.
The definite article is normally used with seasons. Seasons are not capitalized.
¿Cómo es la primavera aquí? What is spring like here?
Months are not capitalized in Spanish.
Mi cumpleaños es en noviembre. My birthday is in November.
Hay veintiocho días en febrero. There are twenty-eight days in February.
In the plural, the days of the week express the idea of doing something regularly.
La clase de filosofía es los lunes, los miércoles y los viernes. Philosophy class is on Mondays, Wednesdays, and Fridays.
Los sábados voy al gimnasio. I go to the gym on Saturdays.
On Monday …, on Tuesday …, etc., is expressed by using the definite article, el or los.
El examen es el lunes. The exam is on Monday.
Days that end in -s have the same form in the singular and the plural.
el lunes los lunes