"Ele não abria a loja aos domingos."

Translation: He used not to open the shop on Sundays.

[Armwood]

why is the preposition "a" before "os domingos"? does it not work with "em" => "nos domingos"?

[Paulenrique]

AOS works for routine to the days sábado e domingo and NAS for the weekdays. Ela não trabalha AOS sábados/domingos.