Definite article in days of the week

The definite article is used before names of the days of the week instead of en to translate the English "on".

Llegarán el viernes. 

They'll arrive on Friday.

Van a la iglesia los domingos. 

They go to church on Sundays. 

The article is not in used after any form of the verb ser. 

Hoy es domingo. 

Today is Sunday. 

Ayer fue sábado. 

Yesterday was Saturday.