"Why Sunday is called Sunday" by Sadiq

This happened in the car when Gulam & the boys were on their way for Chinese class yesterday..:
Sadiq: Papa, I know why Sunday is called Sunday.
Gulam: Why?
Sadiq: Because there is Sun on Sunday.
Gulam: ??

After Gulam told me at night, I asked Sadiq..
Me: Sadiq, there is also Sun on Monday, Tuesday etc, why they are not called Sunday too?
Sadiq : Because Sunday has more Sun.
Me: ok..(Sadiq...Sadiq...)