"As it was on Sunday, stores were not open."
Translation:Comme on était dimanche, les magasins n'étaient pas ouverts.
this is not the translation given in my answer. It says : "comme c'était dimanche... This doesn't look correct to me. I think it should be "comme c'était le dimanche..." which agrees with what Google Translate says.
No, Comme c'était dimanche feels more natural (I'm a native speaker). If you say Comme c'était le dimanche, it feels like it's a specific Sunday.