Los marteses voy a la biblioteca. Is this a contradiction to the rule i.e. "Days of the week do not have a plural form except sabados y domingos."

Hola Ronald,

Yes, the days of the week only have one form for both singular and plural (except for sábado and domingo). You don't need to add -es, so:

Los lunes juego al baloncesto. (On Mondays I play basketball) - plural

Cada lunes juego al baloncesto. ( Each Monday I play basketball) - singular

Saludos

Inma

Hola Ronald - if you look the -ES ending of the "wrong example" is crossed out. I'll add "this is wrong" before it just to make extra sure students realise we are giving a wrong example  here.

Saludos

Shui