Days of the week der
Always 100% of the 8 days of the week we counted take der.
All days of the week are masculine, like der Tag.
der Sonntag
It's der Sonntag. Sonntag is masculine, plural die Sonntage.
Am Sonntag gehe ich in die Moschee.
On Sunday I go to the mosque.
Always 100% of the 8 days of the week we counted take der.
All days of the week are masculine, like der Tag.
Sonntag der
The last part decides the article: der Tag, so der Sonntag.