Days of the week in Spanish are always masculine.
Read and listen to these examples:
In the examples above, note that we use el in order to say on in English. when we are talking about when things take place.
In these examples, we are still talking about when things take place. Notice how we can also use the plural articles (los). This is used as in "every [day of the week]".
This would be incorrect:
En lunes voy al gimnasio.
En sábados voy al gimnasio.
Days of the week in Spanish are never capitalized!
Unlike in English where all days of the week have a plural form, days of the week in Spanish do not have a plural form, except for sábados and domingos. All the other days of the week indicate the plural only by the plural article.
Los marteses voy a la biblioteca.
On Tuesdays I go to the library.