lunes = Monday
martes = Tuesday
miércoles = Wednesday
jueves = Thursday
viernes = Friday
sábado = Saturday
domingo = Sunday
Notice that the days of the week are not capitalized.
The days of the week are all masculine.
el lunes, el martes-----
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.
Days of the week ending in -s do not change form in the plural. Only the article changes.
el lunes, los lunes
el martes, los martes
el miércoles, los miércoles
el jueves, los jueves
el viernes, los viernes
el sábado, los sábados
el domingo, los domingos
Use the verb ser to express the day. You will soon learn more about this verb. For now, simply realize that the word "es" is a conjugation of that verb, and is the correct verb in this use.
¿Qué día es hoy? = What day is today?
Hoy es lunes = Today is Monday.
Mañana es martes. = Tomorrow is Tuesday.
Notice that the following actions do not occur in the present, but rather in the near future.
Salimos el lunes. = We leave on Monday.
Mañana es domingo. = Tomorrow is Sunday.
In Spanish, the present tense of the indicative is sometimes used to express the near future. English does this too.
Salimos el lunes.We (will) leave on Monday.
Mañana es domingo.Tomorrow (will be) is Sunday.
From: www.studyspanish.com