For anybody else still wondering, this page seems helpful on the subject: http://italian.about.com/library/fare/blfare141a.htm
It's great to see that in the Italian course "on a Sunday" is the more excepted translation. Perhaps the conveners are aware of the lyrics of the Oscar winning film of the sixties "Never on Sunday". The group that sung this famous song had an Italian name! While doing the Spanish course it was suggested that the use of "never on a Sunday" was using poetic licence. Thumbs up to the Duolingo Italian team.